38.2 Fungal Skin Infections: Tinea Corporis, Cruris, Pedis & Onychomycosis

ABFM Family Medicine Certification Study Guide Exam

Track your progress as you read

Finished sections get a check mark.

1 Chapter 1: Exam Structure, Blueprint & Clinical Decision-Making

1.1 Exam Blueprint, FMCLA vs. One-Day Exam & Board Scoring 1.2 Clinical Reasoning, Test-Taking Strategy & Ambiguity Management 1.3 Evidence-Based Medicine & Clinical Practice Guidelines in Primary Care

2 Chapter 2: Healthcare Systems, Practice Management & Quality

2.1 Practice Management, Billing, Coding & Documentation 2.2 Quality Improvement, Patient Safety & Diagnostic Stewardship 2.3 Interprofessional Team-Based Care & Care Coordination

3 Chapter 3: Ethics, Law, Biostatistics & Health Equity

3.1 Medical Ethics, Informed Consent & Surrogate Decision-Making 3.2 Biostatistics, Clinical Epidemiology & Diagnostic Test Interpretation 3.3 Social Determinants of Health, Health Equity & Cultural Humility 3.4 Physician Wellness, Burnout Prevention & Professionalism

4 Chapter 4: Lifestyle Counseling & Behavior Change

4.1 Alcohol Use Screening & Brief Intervention 4.2 Illicit Drug & Cannabis Counseling 4.3 Tobacco & Vaping Cessation Counseling

5 Chapter 5: Nutrition, Physical Activity & Chronic Disease Prevention

5.1 Dietary Counseling for Obesity & Chronic Disease Prevention 5.2 Exercise Prescription & Physical Activity Counseling 5.3 Genetic Risk Assessment & Family History Counseling

6 Chapter 6: Routine Adult Immunizations

6.1 ACIP Adult Respiratory Vaccines: Influenza, COVID-19, RSV & Pneumococcal 6.2 ACIP Adult Non-Respiratory Vaccines: Tdap, Shingrix, HPV & Hep B 6.3 Immunizations in Special Populations: Pregnancy, Asplenia & Immunocompromise

7 Chapter 7: Pediatric Health Maintenance & Surveillance

7.1 Newborn Examination & Early Postnatal Care 7.2 Well-Child Care, Developmental Surveillance & Milestones 7.3 Pediatric & Adolescent Screening: Vision, Hearing, Anemia & Lead 7.4 Pediatric Oral Health, Dental Screening & Fluoride

8 Chapter 8: Behavioral Health & Safety Screening

8.1 Primary Care Depression, Anxiety & Behavioral Health Screening 8.2 Domestic Violence, Intimate Partner Violence & Firearm Safety 8.3 Occupational Health & Recreational Safety Counseling

9 Chapter 9: Adult Preventive Screening: Cardiovascular & Metabolic

9.1 Cardiovascular Disease & Hypertension Screening 9.2 Type 2 Diabetes Screening & Prediabetes Risk Stratification 9.3 Abdominal Aortic Aneurysm & Osteoporosis Screening

10 Chapter 10: Adult Cancer Screening: Breast & Gynecologic

10.1 Breast Cancer Screening & Risk-Assessment 10.2 Cervical Cancer Screening & ASCCP Algorithms 10.3 Skin Cancer Clinical Assessment & Counseling

11 Chapter 11: Adult Cancer Screening: Colorectal, Lung & Prostate

11.1 Colorectal Cancer Screening Modalities & Intervals 11.2 Lung Cancer Screening with Low-Dose CT 11.3 Prostate Cancer Screening & Shared Decision-Making

12 Chapter 12: Sexual Health, Infectious Disease Screening & Contraception

12.1 Sexual Health Counseling & Risk-Reduction Strategies 12.2 STI Screening: Chlamydia, Gonorrhea, Syphilis & Trichomoniasis 12.3 Bloodborne Infectious Disease Screening: HIV & Hepatitis C

13 Chapter 13: Reproductive Life Planning, Lactation & Preoperative Care

13.1 Preconception Planning & Comprehensive Contraception Management 13.2 Lactation Support & Breastfeeding Counseling 13.3 Preoperative Medical Evaluation & Pre-Participation Sports Exam

14 Chapter 14: Emergent Cardiovascular Conditions

14.1 Acute Coronary Syndromes & Myocardial Infarction 14.2 Acute Chest Pain Risk Stratification & Triage 14.3 Acute or Critical Valvular Heart Emergencies

15 Chapter 15: Cardiac Arrhythmias, Heart Failure & Hypertensive Crises

15.1 Acute Atrial & Ventricular Arrhythmias 15.2 Acute Decompensated Heart Failure & Cardiogenic Shock 15.3 Hypertensive Emergency & Urgency Management

16 Chapter 16: Vascular & Thromboembolic Emergencies

16.1 Pulmonary Embolism & Deep Vein Thrombosis 16.2 Acute Aortic Syndromes & Peripheral Vascular Emergencies 16.3 Critical Limb Ischemia & Acute Arterial Occlusion

17 Chapter 17: Neurologic Emergencies: Stroke & Altered Consciousness

17.1 Acute Ischemic Stroke & Intracranial Hemorrhage 17.2 Syncope Evaluation & Status Epilepticus 17.3 Acute Delirium, Encephalopathy & Severe Confusion

18 Chapter 18: Neurologic Trauma & Acute Cranial Emergencies

18.1 Concussion, Moderate-to-Severe TBI & Intracranial Injury 18.2 Thunderclap & Acute Severe Secondary Headaches 18.3 Acute Ophthalmologic Emergencies

19 Chapter 19: Acute Abdominal, Pelvic & Gastrointestinal Emergencies

19.1 Acute Abdominal & Pelvic Pain Triage 19.2 Upper & Lower Gastrointestinal Bleeding 19.3 Acute Hepatic Failure, Cholangitis & Severe Jaundice

20 Chapter 20: Acute Pulmonary & Airway Emergencies

20.1 Acute Hypoxic Respiratory Failure & ARDS 20.2 Severe Community-Acquired Pneumonia & Empyema 20.3 Acute Severe Epistaxis & Airway Obstruction

21 Chapter 21: Acute Traumatic Injuries & Wound Care

21.1 Major Trauma Resuscitation & Initial Assessment 21.2 Burn Assessment, Inhalation Injury & Resuscitation 21.3 Laceration Management, Complex Wounds & Animal Bites

22 Chapter 22: Acute Orthopedic & Musculoskeletal Emergencies

22.1 Acute Ambulatory Fractures & Initial Splinting 22.2 Compartment Syndrome & Major Orthopedic Trauma 22.3 Septic Arthritis & Acute Microcrystalline Crises

23 Chapter 23: Severe Infections, Sepsis & Systemic Emergencies

23.1 Sepsis, Septic Shock & Initial Fluid Resuscitation 23.2 Necrotizing Soft Tissue Infections & Severe CNS Infections 23.3 Anaphylaxis Diagnosis, Epinephrine Dosing & Airway Protection

24 Chapter 24: Metabolic, Endocrine & Electrolyte Emergencies

24.1 Diabetic Ketoacidosis & Hyperosmolar Hyperglycemic State 24.2 Critical Electrolyte Emergencies: Dyskalemias & Dysnatremias 24.3 Severe Calcium Abnormalities & Acute Adrenal Crisis

25 Chapter 25: Hematologic, Urologic & Dental Emergencies

25.1 Severe Symptomatic Anemias & Coagulopathic Emergencies 25.2 Testicular Torsion, Priapism & Acute Urinary Retention 25.3 Urgent Odontogenic Infections & Dental Trauma

26 Chapter 26: Psychiatric, Behavioral & Obstetric/Gynecologic Emergencies

26.1 Suicidal Ideation, Acute Agitation & Behavioral Emergencies 26.2 Acute Physical & Sexual Assault Care 26.3 Life-Threatening Vaginal Bleeding & Ectopic Pregnancy 26.4 Environmental Heat Stroke & Severe Hypothermia

27 Chapter 27: Acute Head, Neck & Neurologic Symptoms

27.1 Primary Acute Headaches: Migraine, Tension & Cluster 27.2 Concussion & Minor Closed Head Injury Evaluation 27.3 Transient Ischemic Attack Evaluation & ABCD2 Risk Scoring

28 Chapter 28: Acute Cranial Nerves, Hearing & Sensory Symptoms

28.1 Peripheral Neuropathies: Bell's Palsy, Carpal Tunnel & Radiculopathy 28.2 Acute Tinnitus, Sudden Sensorineural Hearing Loss & Cerumen Impaction 28.3 Acute Dizziness, Benign Paroxysmal Positional Vertigo & Vestibular Neuritis

29 Chapter 29: Acute Upper Respiratory Tract Infections

29.1 Acute Viral Rhinosinusitis vs. Bacterial Rhinosinusitis 29.2 Acute Pharyngitis: Group A Streptococcus & Centor Criteria 29.3 Acute Otitis Media, Otitis Externa & Serous Effusion

30 Chapter 30: Acute Lower Respiratory Infections & Cough

30.1 Influenza, COVID-19 & Viral Respiratory Syndromes 30.2 Acute Bronchitis & Lower Respiratory Tract Infections 30.3 Ambulatory Community-Acquired Pneumonia

31 Chapter 31: Allergic, Nasal & Non-Infectious Pulmonary Conditions

31.1 Allergic Rhinitis & Acute Non-Anaphylactic Allergic Reactions 31.2 Acute Dyspnea in Ambulatory Care 31.3 Interstitial Lung Diseases & Sarcoidosis Presentation

32 Chapter 32: Acute Chest, Cardiac & Vascular Symptoms

32.1 Palpitations Evaluation & Arrhythmia Triage in Primary Care 32.2 Deep Vein Thrombosis & Superficial Thrombophlebitis 32.3 Ambulatory Hypotension & Orthostasis Evaluation

33 Chapter 33: Acute Upper Gastrointestinal Conditions

33.1 Peptic Ulcer Disease & Gastritis 33.2 Dysphagia, Esophagitis & Acute Esophageal Disorders 33.3 Acute Nausea, Vomiting & Foodborne Illness

34 Chapter 34: Acute Lower Gastrointestinal Conditions

34.1 Acute Infectious Diarrhea & Gastroenteritis 34.2 Acute Constipation & Functional Bowel Changes 34.3 Ano-Rectal Disorders: Hemorrhoids, Fissures, Abscesses & Pruritus Ani

35 Chapter 35: Inflammatory, Functional & Malabsorptive GI Disorders

35.1 Acute Flares of Inflammatory Bowel Disease 35.2 Irritable Bowel Syndrome: Diagnosis, Subtyping & Management 35.3 Celiac Disease & Malabsorption Syndromes

36 Chapter 36: Acute Abdominal, Hepatic & Biliary Disorders

36.1 Ambulatory Abdominal & Pelvic Pain Triage 36.2 Acute Cholecystitis, Biliary Colic & Pancreatitis 36.3 Acute Liver Injury & Viral Hepatitis

37 Chapter 37: Acute Abdominal Wall, Hernias & Dental Conditions

37.1 Abdominal Wall, Inguinal & Ventral Hernias 37.2 Acute Dental Pain, Odontogenic Abscess & Aphthous Ulcers 37.3 Acute Edema: Unilateral vs. Bilateral Diagnostic Evaluation

38 Chapter 38: Acute Infectious Dermatoses

38.1 Bacterial Skin Infections: Impetigo, Cellulitis & Cutaneous Abscesses 38.2 Fungal Skin Infections: Tinea Corporis, Cruris, Pedis & Onychomycosis 38.3 Viral Cutaneous Infections: Herpes Simplex, Zoster & Molluscum Contagiosum

39 Chapter 39: Inflammatory, Allergic & Autoimmune Dermatoses

39.1 Atopic Dermatitis, Contact Dermatitis & Eczematous Flares 39.2 Non-Infectious Rashes: Urticaria, Drug Eruptions & Erythema Multiforme 39.3 Cutaneous Malignancies & Premalignant Lesions: AK, BCC, SCC & Melanoma

40 Chapter 40: Acute Shoulder, Elbow & Upper Extremity Injuries

40.1 Rotator Cuff Pathology, Tendinopathy & Shoulder Impingement 40.2 Lateral & Medial Epicondylitis, Olecranon Bursitis & Elbow Sprains 40.3 Wrist & Hand Soft-Tissue Injuries: De Quervain, TFCC & Finger Sprains

41 Chapter 41: Acute Knee, Ankle & Lower Extremity Injuries

41.1 Meniscal Tears, Ligamentous Knee Injuries & Patellofemoral Pain 41.2 Ankle Sprains, Ottawa Ankle Rules & Achilles Tendinopathy 41.3 Plantar Fasciitis, Shin Splints & Overuse Syndromes

42 Chapter 42: Acute Ambulatory Fractures & Dislocations

42.1 Distal Radius, Scaphoid & Common Upper Extremity Fractures 42.2 Metatarsal, Clavicle & Common Lower Extremity Fractures 42.3 Joint Dislocation Management: Nursemaid's Elbow & Shoulder Subluxation

43 Chapter 43: Acute Axial Musculoskeletal Pain & Falls

43.1 Acute Low Back Pain: Red Flags, Strain & Disc Herniation 43.2 Acute Cervical Strain, Whiplash & Axial Neck Pain 43.3 Evaluation of Falls in Ambulatory & Older Patients

44 Chapter 44: Acute Inflammatory Arthropathies & Generalized Pain

44.1 Acute Inflammatory Arthritis & Spondyloarthropathies 44.2 Myalgias, Rhabdomyolysis & Post-Viral Pain Syndromes 44.3 Acute Non-Headache Pain Management Strategies

45 Chapter 45: Acute Renal, Urinary & Genitourinary Conditions

45.1 Uncomplicated UTI, Pyelonephritis & Acute Urethritis 45.2 Hematuria Workup & Glomerulonephritis 45.3 Nephrolithiasis & Acute Urinary Tract Obstruction

46 Chapter 46: Acute Male Genitourinary & Prostatic Conditions

46.1 Acute Prostatitis, Epididymitis & Orchitis 46.2 Acute Scrotal Pain: Torsion vs Appendage, BPH & Male Genital Emergencies 46.3 Acute Urinary Retention & Incontinence Evaluation

47 Chapter 47: Acute Women's Health & Early Pregnancy Care

47.1 Early Pregnancy Diagnosis, Dating & Initial Complications 47.2 Abnormal Uterine Bleeding & Acute Menstrual Disorders 47.3 Premenstrual Syndrome & Premenstrual Dysphoric Disorder

48 Chapter 48: Breast Disorders, STIs & Pediatric Elimination

48.1 Palpable Breast Mass, Mastitis & Nipple Discharge 48.2 Sexually Transmitted Infections: Diagnosis, Staging & CDC Guidelines 48.3 Pediatric Enuresis & Encopresis

49 Chapter 49: Systemic Signs, Metabolic Dysregulation & Miscellaneous Infections

49.1 Acute Fever, Sepsis Screening & FUO in Primary Care 49.2 Acute Hyperglycemia & Symptomatic Hypoglycemia 49.3 Electrolyte Abnormalities: Mild-to-Moderate Disturbances 49.4 Vector-Borne, Tick-Borne & Rickettsial Illnesses

50 Chapter 50: Chronic Cardiovascular Diseases: Hypertension & Ischemia

50.1 Essential Hypertension Management: 2025 AHA/ACC Guideline & JNC Strategies 50.2 Chronic Coronary Artery Disease & Post-MI Secondary Prevention 50.3 Chronic Heart Failure: HFrEF, HFpEF & Quadruple GDMT

51 Chapter 51: Chronic Vascular, Valvular & Dyslipidemic Diseases

51.1 Peripheral Arterial Disease & Claudication 51.2 Chronic Valvular Heart Disease & Murmur Evaluation 51.3 Lipid Disorders & ASCVD Risk-Based Statin Therapy

52 Chapter 52: Chronic Respiratory Diseases: Asthma & COPD

52.1 Chronic Asthma Management: GINA Stepwise SMART Therapy 52.2 Chronic Obstructive Pulmonary Disease: GOLD Recommendations 52.3 Solitary Pulmonary Nodule Evaluation & Fleischner Society Criteria

53 Chapter 53: Chronic Metabolic Diseases: Diabetes Mellitus

53.1 Type 2 Diabetes Management: ADA Pharmacotherapy Algorithms 53.2 Type 1 Diabetes: Basal-Bolus Insulin, CGMs & Diabetic Complications 53.3 Diabetic Microvascular & Macrovascular Disease Surveillance

54 Chapter 54: Obesity, Nutrition & Metabolic Bone Diseases

54.1 Medical & Behavioral Management of Obesity & Incretin Mimetics 54.2 Osteoporosis & Metabolic Bone Disease: DEXA, FRAX & Antiresorptives 54.3 Nutritional Deficiencies: Vitamin D, B12, Iron & Malnutrition

55 Chapter 55: Chronic Endocrine Disorders

55.1 Primary & Subclinical Hypothyroidism, Hyperthyroidism & Nodules 55.2 Adrenal Disorders: Addison's Disease & Cushing's Syndrome 55.3 Hypercalcemia, Hyperparathyroidism & Pituitary Disorders

56 Chapter 56: Chronic Kidney Disease & Venous Disorders

56.1 Chronic Kidney Disease: KDIGO Staging & Renoprotective Therapy 56.2 Chronic Venous Insufficiency, Stasis Dermatitis & Lymphedema 56.3 Chronic Electrolyte & Acid-Base Management in CKD

57 Chapter 57: Chronic Gastrointestinal & Hepatic Diseases

57.1 Gastroesophageal Reflux Disease, Barrett's Esophagus & Dyspepsia 57.2 Chronic Liver Disease: MASLD/NAFLD, Cirrhosis & Portal Hypertension 57.3 Viral Hepatitis B & C: Long-Term Monitoring & DAA Therapy

58 Chapter 58: Colorectal Surveillance & Chronic Pancreaticobiliary Care

58.1 Post-Polypectomy Colorectal Polyp Surveillance Guidelines 58.2 Chronic Pancreatitis & Exocrine Pancreatic Insufficiency 58.3 Chronic Nausea, Gastroparesis & Functional GI Disorders

59 Chapter 59: Chronic Musculoskeletal & Rheumatologic Disorders

59.1 Osteoarthritis: Hip, Knee & Hand Management 59.2 Spinal Conditions: Spinal Stenosis, Spondylolisthesis & Compression Fractures 59.3 Chronic Myopathies & Fibromyalgia Syndrome

60 Chapter 60: Chronic Neurologic & Neurodegenerative Disorders

60.1 Dementia: Alzheimer's, Vascular & Lewy Body Disease 60.2 Parkinson's Disease & Essential Tremor 60.3 Epilepsy: Chronic Antiseizure Medications & Post-Stroke Care

61 Chapter 61: Chronic Peripheral Nerves, Pain & Sensory Disorders

61.1 Diabetic & Chronic Peripheral Neuropathies 61.2 Chronic Pain Syndromes & Chronic Daily Headaches 61.3 Chronic Tinnitus, Presbycusis & Persistent Dizziness

62 Chapter 62: Chronic Psychiatric Disorders

62.1 Major Depressive Disorder & Generalized Anxiety Disorder 62.2 Bipolar Disorder: Maintenance Pharmacotherapy & Lithium Monitoring 62.3 Adult & Pediatric Attention-Deficit/Hyperactivity Disorder

63 Chapter 63: Chronic Behavioral Health & Neurodevelopmental Conditions

63.1 Chronic Behavioral Conditions: PTSD, OCD & Somatization 63.2 Autism Spectrum Disorder & Intellectual Disabilities 63.3 Eating Disorders: Anorexia Nervosa, Bulimia & Binge Eating

64 Chapter 64: Chronic Substance Use Disorders & Tobacco Cessation

64.1 Alcohol Use Disorder: Pharmacotherapy & Relapse Prevention 64.2 Opioid & Stimulant Use Disorders: MOUD & Harm Reduction 64.3 Longitudinal Tobacco Cessation Pharmacotherapy

65 Chapter 65: Chronic Dermatologic Conditions

65.1 Acne Vulgaris, Rosacea & Seborrheic Dermatitis 65.2 Chronic Plaque Psoriasis & Biologic Therapies 65.3 Chronic Eczema, Hidradenitis Suppurativa & Cutaneous Lupus

66 Chapter 66: Chronic Reproductive, Infectious & Geriatric Special Care

66.1 Chronic Menstrual Disorders, Infertility & Sexual Dysfunction 66.2 Menopause Management & Hormone Replacement Therapy 66.3 Chronic HIV Infection Management & Primary Care Integration 66.4 Gender-Affirming Primary Care & Hormone Therapy 66.5 Advance Care Planning, Palliative Care & End-of-Life Symptom Control

67 Chapter 67: Continuity Care: Sleep, Fatigue, Cancer, Pregnancy & the Adult Preventive Visit

67.1 Chronic Insomnia & Obstructive Sleep Apnea 67.2 Chronic Malaise, Fatigue & ME/CFS 67.3 Primary Care During Cancer Treatment & Survivorship 67.4 Routine Prenatal Care & Longitudinal Pregnancy Management 67.5 The Periodic Adult Health Examination & Medicare Annual Wellness Visit

38.2 Fungal Skin Infections: Tinea Corporis, Cruris, Pedis & Onychomycosis

Key Takeaways Explain This Topic AI

Dermatophytes (Trichophyton, Microsporum, Epidermophyton) are keratinophilic filamentous fungi that colonize stratum corneum, hair shafts, and nail plates; potassium hydroxide (KOH) examination reveals pathognomonic branching, septate hyphae with uniform caliber.

Tinea corporis, tinea cruris, and tinea pedis respond effectively to topical allylamines (terbinafine) or azoles; combination steroid-antifungal preparations (e.g., clotrimazole-betamethasone) must be avoided due to the high risk of producing 'tinea incognito' and severe steroid-induced dermal atrophy.

Tinea capitis requires mandatory systemic oral antifungal therapy because topical agents cannot penetrate deep hair follicles; oral griseofulvin (with fatty food for 6-8 weeks) is preferred for Microsporum, whereas oral terbinafine (4-6 weeks) is first-line for Trichophyton tonsurans.

Onychomycosis mandates laboratory mycological confirmation (KOH preparation, fungal culture, or nail clipping PAS histology) prior to initiating prolonged oral therapy due to the hepatotoxic potential of oral terbinafine and because over 50% of dystrophic nails are non-fungal in etiology.

First-line systemic therapy for dermatophytic onychomycosis is oral terbinafine 250 mg daily for 6 weeks (fingernails) or 12 weeks (toenails), with baseline hepatic transaminase testing; tinea versicolor is caused by Malassezia yeast ('spaghetti and meatballs' on KOH) and is treated with topical selenium sulfide 2.5% or ketoconazole 2% shampoo.

Last updated: September 2026

Biology & Pathogenesis of Dermatophytoses

Dermatophytoses (commonly designated as "tinea" infections followed by the anatomical site) represent superficial fungal infections caused by a specialized group of filamentous fungi called dermatophytes . Dermatophytes are taxonomically classified into three primary genera: Trichophyton (e.g., T. rubrum , T. mentagrophytes , T. tonsurans ), Microsporum (e.g., M. canis ), and Epidermophyton (e.g., E. floccosum ).

Pathophysiologic Mechanisms

Keratinotropism : Dermatophytes are obligate keratinophilic fungi. They possess specialized extracellular proteases ( keratinases ) that digest keratin, allowing the organisms to adhere to, invade, and proliferate within non-viable, keratinized host structures: the stratum corneum of the epidermis, hair shafts and follicles , and nail plates .

Depth of Invasion : In immunocompetent hosts, dermatophytes are restricted strictly to fully cornified tissue. They are incapable of penetrating viable deep epidermal layers or the dermis because they are inhibited by host non-specific serum inhibitory factors (transferrin) and cannot survive at core human body temperature (≥37°C).

Transmission : Dermatophytes are transmitted through direct contact with infected humans (anthropophilic species, e.g., T. rubrum , T. tonsurans ), animals (zoophilic species, e.g., M. canis from cats and dogs), or contaminated soil (geophilic species). Anthropophilic infections tend to cause chronic, low-grade, minimally inflammatory disease, whereas zoophilic and geophilic species trigger intense cell-mediated hypersensitivity reactions in human hosts.

Diagnostic Techniques in Cutaneous Mycology

Accurate diagnosis of fungal skin disease relies on bedside direct microscopy and confirmatory laboratory testing. Presumptive visual diagnosis without diagnostic confirmation leads to frequent misdiagnosis, inappropriate corticosteroid prescription, and prolonged clinical morbidity.

1. Potassium Hydroxide (KOH) Direct Microscopy

Sampling Technique : The leading, active, erythematous, scaly border of the cutaneous lesion contains the highest density of viable hyphae. Using a sterile #15 scalpel blade held at a 45-degree angle, the clinician gently scrapes epidermal scales from the advancing margin onto a clean glass microscope slide.

Preparation : A drop of 10% to 20% potassium hydroxide (KOH) solution is applied, and a coverslip is placed over the specimen. The slide is gently warmed over an alcohol flame (without boiling) or allowed to stand at room temperature for 10 to 15 minutes. KOH acts as a powerful keratolytic agent, dissolving host keratin, lipids, and cellular debris while leaving the fungal chitinous cell walls intact.

Microscopic Findings : Under low power (10x) and confirmatory high power (40x), dermatophyte infection is confirmed by the visualization of branching, septate hyphae of uniform caliber (width) traversing squamous cells, often accompanied by chains of spherical arthrospores .

Artifact Pitfalls : Clinicians must distinguish true fungal hyphae from the "mosaic fungus" artifact (an optical illusion formed by lipid droplets and cholesterol crystals aligning along the undulating cell borders of keratinocytes) and cotton fibers (which lack uniform caliber, have irregular branching, and lack true internal septations).

2. Wood's Lamp Examination

A Wood's lamp emits long-wave ultraviolet radiation (peak wavelength ~365 nm) in a completely darkened room.

Dermatophytes : Hairs infected with certain Microsporum species ( Microsporum canis , M. audouinii ) emit a striking, bright blue-green fluorescence due to pteridine metabolites deposited in the hair cortex. Important Pearl : The vast majority of tinea capitis cases in North America today are caused by Trichophyton tonsurans , which does NOT fluoresce under a Wood's lamp!

Other Cutaneous Organisms :

Corynebacterium minutissimum (erythrasma): Emits an intense, pathognomonic coral-red fluorescence due to coproporphyrin III production;

Pseudomonas aeruginosa : Emits a bright yellow-green fluorescence due to pyoverdine synthesis;

Malassezia (tinea versicolor): Emits a faint yellow-gold or copper-orange fluorescence .

3. Fungal Culture & Histopathology

Fungal Culture : Inoculation of scraped scales, hair, or nail fragments onto Sabouraud dextrose agar containing chloramphenicol and cycloheximide. Culture requires 2 to 4 weeks of incubation to identify specific fungal species and determine antifungal susceptibilities.

Periodic Acid-Schiff (PAS) Staining : The gold-standard histopathologic stain for nail plate clippings and skin biopsies. PAS oxidizes 1,2-glycol groups in fungal wall polysaccharides into aldehydes, reacting with Schiff reagent to stain fungal hyphae a vibrant, unmistakable magenta/pink.

Superficial Dermatophytic Syndromes SUPERFICIAL DERMATOPHYTIC INFECTIONS

• Syndrome Primary Pathogens Hallmark Clinical Manifestations
• ─────────────────────────────────────────────────────────────────────────────
• Tinea Corporis T. rubrum, Annular erythematous plaque, raised
• ("Ringworm") T. mentagrophytes, scaly advancing border, central clearing;

M. canis centrifugal expansion.

• ─────────────────────────────────────────────────────────────────────────────
• Tinea Cruris T. rubrum, Pruritic erythematous rash in groin folds
• ("Jock Itch") E. floccosum and inner thighs; ADVANCING SCALY BORDER;

CHARACTERISTICALLY SPARES SCROTUM & PENIS.

• ─────────────────────────────────────────────────────────────────────────────
• Tinea Pedis T. rubrum, Interdigital maceration & fissuring (4th/5th);
• ("Athlete's Foot") T. mentagrophytes Moccasin distribution (scaling on soles/sides);

Vesiculobullous inflammatory flares.

• ─────────────────────────────────────────────────────────────────────────────
• Tinea Capitis T. tonsurans (>90%), Patchy alopecia, scaly scalp, "black dot"
• M. canis hairs, occipital LAD; Kerion (boggy mass);

MANDATORY SYSTEMIC ORAL THERAPY!

• ─────────────────────────────────────────────────────────────────────────────
• Onychomycosis T. rubrum, Subungual hyperkeratosis, yellow-brown
• (Tinea Unguium) T. mentagrophytes discoloration, onycholysis, brittle nail plate;

MANDATORY PRE-TREATMENT CONFIRMATION!

1. Tinea Corporis ("Ringworm")

Clinical Presentation : Begins as a pruritic, circular erythematous macule or papule that spreads centrifugally. As the central area clears, it evolves into the classic annular ("ring-shaped") plaque featuring a raised, active, erythematous, vesicular or scaly advancing border and a relatively clear, hypopigmented or normal central zone.

Topical Therapy : First-line for localized tinea corporis:

Allylamines : Topical Terbinafine 1% cream applied once or twice daily for 1 to 2 weeks. Allylamines inhibit squalene epoxidase, blocking ergosterol synthesis and accumulating toxic squalene; they are fungicidal and associated with lower relapse rates and shorter treatment durations than azoles.

Imidazoles / Azoles : Topical Clotrimazole 1% , Ketoconazole 2% , or Miconazole 2% cream applied twice daily for 2 to 4 weeks. Azoles inhibit 14-alpha-demethylase; they are fungistatic and require application for at least 1 to 2 weeks after visible clinical resolution to eradicate latent fungal elements.

Systemic Therapy : Indicated for extensive body surface area involvement, immunosuppressed hosts, deep follicular penetration ( Majocchi granuloma ), or failure of topical therapy: Oral Terbinafine 250 mg PO daily for 2 to 4 weeks, or Oral Itraconazole 200 mg PO daily for 1 to 2 weeks.

Tinea Incognito Warning : Application of topical corticosteroids (alone or in combination products such as betamethasone dipropionate-clotrimazole [ Lotrisone ]) blunts the local host inflammatory response. This temporarily masks symptoms while allowing fungal hyphae to proliferate unchecked into deeper follicular structures. The rash loses its classic annular raised scaly border, becoming an ill-defined, violaceous, diffuse plaque known as tinea incognito , which is notoriously difficult to diagnose and eradicate.

2. Tinea Cruris ("Jock Itch")

Clinical Presentation : Intensely pruritic, sharply demarcated, symmetrical erythematous rash located in the intertriginous groin creases and extending onto the upper inner thighs. Features an elevated, active, scaling advancing margin.

The Crucial Board Differentiating Feature : Tinea cruris characteristically SPARES the scrotum and penis ! In contrast:

Cutaneous Candidiasis (Intertrigo) : Overwhelmingly involves the scrotum , presenting with bright beefy-red erythema, maceration, and pathognomonic peripheral satellite pustules ;

Erythrasma : Presents as a uniform, reddish-brown, finely wrinkled patch in the groin that lacks an active raised border and exhibits brilliant coral-red fluorescence under Wood's lamp .

Treatment : Topical terbinafine 1% or topical azoles applied twice daily for 2 to 4 weeks. Patients must keep the inguinal folds dry, wear loose-fitting cotton boxers, and concurrently treat coexisting tinea pedis to prevent reinfection.

3. Tinea Pedis ("Athlete's Foot")

Epidemiology : The single most common dermatophytosis worldwide. Fungal transmission occurs via walking barefoot on contaminated floors in communal showers, locker rooms, and pool decks.

Clinical Patterns :

Interdigital Tinea Pedis : Most frequent pattern. Features maceration, erythema, peeling, and painful fissuring in the toe web spaces, particularly between the fourth and fifth toes .

Moccasin Type (Chronic Hyperkeratotic) : Features chronic, diffuse, fine, silvery-white scaling and bilateral hyperkeratotic thickening distributed over the soles, heels, and lateral borders of both feet in a "moccasin" distribution. Commonly manifests as the classic "two feet, one hand syndrome" (chronic tinea pedis involving both feet accompanied by unilateral tinea manuum on the dominant hand).

Vesiculobullous (Inflammatory) : Acute eruption of intensely pruritic, painful vesicles or bullae on the plantar instep, often triggered by warmth.

Clinical Significance : Interdigital tinea pedis creates microscopic disruptions in the cutaneous barrier, serving as the primary anatomical portal of bacterial entry for recurrent lower-extremity non-purulent cellulitis and erysipelas !

Management : Interdigital disease responds to topical terbinafine 1% cream BID for 1 to 2 weeks. Moccasin-type tinea pedis frequently fails topical monotherapy due to the thick hyperkeratotic stratum corneum and requires Oral Terbinafine 250 mg daily for 2 to 4 weeks combined with topical keratolytic agents ( salicylic acid 6% or urea 20-40% cream ) to thin the cornified layer.

Tinea Capitis & Kerion

Tinea capitis is a dermatophyte infection of scalp hair follicles and hair shafts, occurring almost exclusively in prepubertal children (peak incidence ages 3 to 9 years, with highest prevalence in African American children).

Pathophysiology & Hair Shaft Invasion

Causative Organisms : In the United States and North America, >90% of cases are caused by Trichophyton tonsurans (anthropophilic, spread person-to-person via shared combs, brushes, pillows, hats). Zoophilic cases are caused by Microsporum canis (transmitted from kittens or puppies).

Patterns of Hair Invasion :

Endothrix : Fungal arthroconidia proliferate entirely within the interior of the hair shaft, destroying the internal keratin matrix. The weakened hair breaks off flush at the level of the follicular scalp orifice, producing the pathognomonic physical finding of "black dot" tinea capitis (classic for T. tonsurans );

Ectothrix : Arthroconidia form a sheath around the external surface of the hair shaft, breaking hairs a few millimeters above the scalp (characteristic of M. canis ).

Clinical Presentation

Presents as one or multiple patches of scaling, pruritic alopecia studded with broken hair stubs ("black dots").

Prominent, tender posterior cervical and suboccipital lymphadenopathy is an exceptionally sensitive clinical hallmark distinguishing tinea capitis from non-infectious causes of pediatric hair loss (e.g., alopecia areata, trichotillomania).

Kerion : A severe, aggressive, cell-mediated hypersensitivity reaction to the dermatophyte. Presents as a markedly elevated, boggy, fluctuant, purulent, inflammatory granulomatous tumefaction studded with open pustules, draining sinus tracts, and thick crusts. Critical Board Pearl : A kerion is frequently misdiagnosed as a bacterial scalp abscess or carbuncle , leading to inappropriate surgical incision! Incision is strictly contraindicated and ineffective because the process is hypersensitivity-driven rather than a bacterial pocket. If untreated, a kerion causes permanent cicatricial (scarring) alopecia.

Mandatory Systemic Therapy for Tinea Capitis

Important

THE CARDINAL RULE OF TINEA CAPITIS: TOPICAL THERAPY CANNOT CURE SCALP RINGWORM! Topical antifungal creams, lotions, and shampoos cannot penetrate down into the hair follicle bulb or enter the interior of the hair shaft . Therefore, SYSTEMIC ORAL ANTIFUNGAL THERAPY IS MANDATORY for the treatment of all tinea capitis cases! Topical antifungal shampoos (selenium sulfide 2.5% or ketoconazole 2%) are used strictly as adjunctive therapy (applied twice weekly) to decrease viable spore shedding, reducing secondary transmission to family members and classmates.

First-Line Oral Regimens for Tinea Capitis

Oral Griseofulvin :

Dosing : Microsize formulation at 20 to 25 mg/kg/day PO (or ultramicrosize formulation at 10 to 15 mg/kg/day) once daily or divided BID for 6 to 8 weeks .

Administration Rule : Griseofulvin is highly lipophilic and has poor aqueous absorption. It MUST be ingested with a high-fat meal or beverage (whole milk, peanut butter, ice cream, yogurt) to ensure adequate gastrointestinal absorption and therapeutic tissue concentrations.

Clinical Indication : Historically the gold-standard agent; remains the drug of choice for Microsporum canis infections.

Adverse Effects : Gastrointestinal distress, headache, drug eruptions, photosensitivity, and rare hepatotoxicity. Avoid in patients with porphyria or severe hepatic disease.

Oral Terbinafine :

Dosing : Weight-based once-daily dosing for 4 to 6 weeks :

• Children <20 kg: 62.5 mg PO once daily;
• Children 20 to 40 kg: 125 mg PO once daily;

Children >40 kg: 250 mg PO once daily.

Clinical Indication : The preferred first-line agent for Trichophyton tonsurans due to its shorter treatment duration (4-6 weeks vs. 6-8 weeks for griseofulvin) and superior mycological cure rates.

Management of Kerion :

Initiate systemic oral antifungal therapy (terbinafine or griseofulvin) immediately.

Add an adjunctive course of Oral Prednisone (1 mg/kg/day, maximum 40 mg daily) for 10 to 14 days to rapidly suppress the destructive cell-mediated hypersensitivity reaction, relieving severe pain and preventing permanent cicatricial scarring alopecia.

Onychomycosis (Tinea Unguium)

Onychomycosis is a fungal infection of the nail unit (nail bed, nail plate, and nail matrix), accounting for roughly 50% of all nail disorders in adults. Its prevalence increases with advancing age, diabetes mellitus, peripheral vascular disease, and immunosuppression.

Etiology & Clinical Subtypes

Over 85% to 90% of cases are caused by dermatophytes, overwhelmingly Trichophyton rubrum , followed by Trichophyton mentagrophytes . Non-dermatophyte molds ( Fusarium , Acremonium ) and Candida species account for the remainder.

Distal Lateral Subungual Onychomycosis (DLSO) : The most frequent clinical pattern. Dermatophytes invade the hyponychium and distal nail bed, spreading proximally. Clinical features include:

• Subungual hyperkeratosis (accumulation of thick, cheesy keratinaceous debris under the nail plate);
• Onycholysis (separation of the nail plate from the underlying pink nail bed);
• Yellowish-brown, chalky discoloration of the nail plate;

Brittleness, crumbling, and progressive nail plate dystrophy.

Mandatory Pre-Treatment Mycological Confirmation ONCHOMYCOSIS: MANDATORY CONFIRMATION PROTOCOL

• Suspected Onychomycosis (Thickened, Dystrophic, Discolored Nail Plate)
• │
• ▼
• MANDATORY PRE-TREATMENT CONFIRMATION TESTING

• Up to 50% of dystrophic nails are NOT fungal!

• (Differential: Psoriasis, Lichen Planus, Trauma)
• Oral Terbinafine requires 6-12 weeks of treatment
• Risk of drug-induced liver injury & CYP450 drug interactions
• │
• ▼

SELECT DIAGNOSTIC MODALITY (Order at least one before treating):

• [1] PAS Histology of Nail Clipping (Highest sensitivity: 80-90%)
• [2] KOH Direct Microscopy of subungual scraping (Rapid bedside)
• [3] Fungal Culture on Sabouraud agar (Identifies specific species)
• │

Is Fungal Etiology Confirmed?

• ╱ ╲
• NO YES
• ╱ ╲
• ▼ ▼

DO NOT PRESCRIBE ORAL TERBINAFINE! CONFIRMED ONYCHOMYCOSIS:

• Re-evaluate differential: • Baseline LFTs (ALT/AST)
• Nail psoriasis (pitting, oil drop)• Oral Terbinafine 250 mg daily
• Lichen planus, repetitive trauma • Fingernails: 6 weeks
• Repeat biopsy / nail clipping • Toenails: 12 weeks

Caution

CRITICAL BOARD & SAFETY RULE: NEVER TREAT UNCONFIRMED ONYCHOMYCOSIS! Up to 50% of thickened, discolored, dystrophic nails are NOT caused by fungal infections ! Common non-fungal etiologies include nail psoriasis (characterized by nail pitting, salmon/oil-drop patches, and onycholysis), lichen planus (longitudinal ridging and pterygium), chronic microtrauma (from poorly fitting footwear), chronic eczema, and peripheral arterial disease. Because systemic antifungal therapy requires prolonged administration (6 to 12 weeks) and carries significant risks of drug-induced liver injury (severe hepatotoxicity) , adverse taste disturbances, and potent CYP450 drug interactions, CLINICIANS MUST DEFIRMATIVELY CONFIRM A FUNGAL INFECTION (via PAS stain, KOH, or fungal culture) BEFORE PRESCRIBING SYSTEMIC ANTIFUNGAL THERAPY !

Evidence-Based Treatment Regimens for Onychomycosis

Oral Terbinafine (First-Line Systemic Gold Standard) :

Dosing : 250 mg PO once daily for 6 weeks (fingernails) or 12 weeks (toenails) .

Efficacy : Highest mycological cure rate (~70% to 75%) and clinical cure rate (~40% to 50%).

Laboratory Monitoring : Check baseline liver function tests (serum ALT and AST) prior to treatment initiation. In patients with pre-existing hepatic disease or prolonged therapy, repeat transaminases midway through treatment. Counsel patients to immediately discontinue the drug and report symptoms of hepatitis: persistent nausea, fatigue, right upper quadrant abdominal pain, dark urine, or jaundice.

Contraindications : Active or chronic liver disease, severe renal insufficiency (creatinine clearance <50 mL/min).

Drug Interactions : Terbinafine is a potent inhibitor of the cytochrome P450 CYP2D6 isoenzyme; co-administration increases serum levels of tricyclic antidepressants, beta-blockers, and SSRIs.

Oral Itraconazole (Alternative Systemic Agent) :

Pulse Dosing : 200 mg PO BID for 1 week per month; 2 pulses for fingernails, 3 pulses for toenails.

Precautions : Potent inhibitor of CYP3A4 (numerous drug-drug interactions with statins, direct oral anticoagulants, calcium channel blockers). Boxed warning: Negative inotropic effect—strictly contraindicated in patients with congestive heart failure or left ventricular dysfunction.

Topical Antifungal Therapies :

Agents include Ciclopirox 8% nail lacquer applied daily for 48 weeks, Efinaconazole 10% solution daily for 48 weeks, and Tavaborole 5% solution daily for 48 weeks.

Limitations : Modest efficacy (complete cure rates only 8% to 18%) due to poor penetration through the dense keratin nail plate.

Indications : Reserved for mild-to-moderate superficial distal disease involving <50% of the nail plate without lunula/matrix involvement, or when oral antifungals are contraindicated.

Patient Expectation Counseling : Even after completing the full 12-week course of oral terbinafine, the toenail will not look cosmetically normal immediately . Because human toenails grow at a rate of only 1 to 1.5 mm per month, a fully normal, clear nail plate requires 9 to 18 months to completely grow out .

Loading diagram... Diagnostic and Therapeutic Flowchart for Cutaneous Fungal Infections