Section 12.1: Common Dermatologic Presentations

USMLE Step 3 Exam

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1 Foundations - Biostatistics & Study Design

1.1 Study Designs & Clinical Trials 1.2 Hypothesis Testing & Statistical Significance 1.3 Measures of Association & Diagnostic Accuracy

2 Foundations - Epidemiology & Literature Interpretation

Section 2.1: Epidemiology & Risk Measures Section 2.2: Interpretation of Medical Literature & Ads Section 2.3: Preventive Medicine & Screening Guidelines

3 Foundations - Ethics & Communication

3.1 Medical Ethics & Patient Autonomy 3.2 Professionalism & Communication Skills 3.3 Legal Medicine & Informed Consent

4 Foundations - Patient Safety & Systems

Section 4.1: Patient Safety & Error Reduction Section 4.2: Quality Improvement & Systems-Based Practice Section 4.3: Healthcare Delivery Systems & Transitions of Care

5 Cardiovascular Management

5.1 Ischemic Heart Disease & Myocardial Infarction 5.2 Heart Failure & Cardiomyopathies 5.3 Arrhythmias & Conduction System Disorders 5.4 Vascular Diseases & Hypertension Management

6 Respiratory Management

Section 6.1: Obstructive Lung Diseases Section 6.2: Restrictive & Interstitial Lung Diseases Section 6.3: Infectious Pulmonary Diseases Section 6.4: Pulmonary Vascular & Pleural Diseases

7 Gastrointestinal & Hepatic Management

Esophageal & Gastric Disorders Intestinal & Colorectal Disorders Hepatic, Biliary, & Pancreatic Diseases Nutritional & Malabsorptive Syndromes

8 Renal, Urinary, & Endocrine Management

Section 8.1: Acute Kidney Injury & Chronic Kidney Disease Section 8.2: Fluid, Electrolyte, & Acid-Base Disorders Section 8.3: Diabetes Mellitus & Metabolic Syndrome Section 8.4: Thyroid, Adrenal, & Pituitary Disorders

9 Neurology, Special Senses, & Behavioral Health

9.1 Cerebrovascular Disease & Stroke 9.2 Neurodegenerative & Demyelinating Disorders 9.3 Psychiatric Emergencies & Substance Use Disorders 9.4 Common Mood, Anxiety, & Psychotic Disorders

10 Obstetrics & Gynecology

10.1 Antepartum Care & Pregnancy Complications 10.2 Intrapartum Management & Postpartum Care 10.3 Gynecology & Breast Disorders

11 Pediatrics, Human Development, & Multisystem Care

11.1 Normal Development & Childhood Immunizations 11.2 Common Pediatric & Neonatal Illnesses 11.3 Multisystem & Rheumatologic Diseases

12 Skin, Musculoskeletal, & CCS Case Management

Section 12.1: Common Dermatologic Presentations Section 12.2: Acute & Chronic Musculoskeletal Injuries Section 12.3: Computer-Based Case Simulations (CCS) Strategy

Section 12.1: Common Dermatologic Presentations

Key Takeaways Explain This Topic AI

Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) are CD8+ T-cell-mediated hypersensitivity reactions distinguished by the percentage of skin detachment (<10% for SJS, >30% for TEN) and require immediate drug cessation and supportive care in a burn unit.

Erysipelas is a superficial dermal infection characterized by a bright red, shiny plaque with sharp, raised borders and is caused by Streptococcus pyogenes, whereas cellulitis is a deeper dermal infection with flat, poorly demarcated borders caused by S. pyogenes and S. aureus.

Herpes zoster presents as a painful, unilateral vesicular eruption in a dermatomal distribution; vesicles on the tip of the nose (Hutchinson sign) indicate Herpes Zoster Ophthalmicus and require urgent ophthalmology referral to prevent corneal damage.

Melanoma screening uses the ABCDE criteria and the ugly duckling sign; suspected lesions must be evaluated with a full-thickness excisional biopsy with 1-3 mm margins rather than a superficial shave to ensure accurate Breslow thickness measurement.

Last updated: July 2026

Common Dermatologic Presentations: SJS/TEN, Cellulitis vs. Erysipelas, Zoster, and Skin Cancers

Dermatology in the USMLE Step 3 exam focuses on critical triage, distinguishing emergent conditions from benign cutaneous lesions, and selecting appropriate therapies. Clinicians must identify life-threatening drug reactions, localize skin infections, manage herpes zoster complications, and systematically evaluate pigmented lesions for malignancy.

Severe Cutaneous Adverse Reactions: SJS and TEN

Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) represent life-threatening, T-cell-mediated cytotoxic hypersensitivity reactions characterized by widespread keratinocyte apoptosis. The pathophysiology involves CD8+ cytotoxic T-lymphocytes releasing granulysin and Fas-ligand, leading to full-thickness epidermal necrosis. The distinction is determined by the percentage of total body surface area (BSA) showing epidermal detachment: SJS involves less than 10% BSA; SJS/TEN overlap involves 10% to 30%; and TEN involves greater than 30% BSA.

Patients present with a prodrome of high fever, malaise, and painful swallowing, followed by the rapid development of flaccid bullae and a positive Nikolsky sign (epidermal detachment with lateral pressure). Mucosal involvement (erythema, painful erosions) is present in over 90% of cases and must involve at least two distinct sites (e.g., oral, ocular, urogenital). High-risk medications are the most common trigger, summarized by the SATAN mnemonic: Sulfonamides, Allopurinol, Tetracyclines/Anticonvulsants (carbamazepine, lamotrigine, phenytoin), NSAIDs (meloxicam, piroxicam), and Nevirapine. Mycoplasma pneumoniae is a common infectious trigger.

An urgent skin biopsy confirms the diagnosis, demonstrating full-thickness necrosis of the epidermis with subepidermal cleavage and minimal dermal inflammation. Management requires immediate cessation of all suspected drugs and transfer to a specialized burn unit or ICU for supportive care. Ocular involvement can lead to symblepharon and blindness; thus, an urgent ophthalmology consult is mandatory.

Cellulitis versus Erysipelas

Distinguishing cellulitis from erysipelas relies on identifying the depth of infection and the margins of the skin lesion. Erysipelas is a superficial infection of the upper dermis and superficial lymphatics, almost exclusively caused by Group A Streptococcus (Streptococcus pyogenes). It presents with a rapid onset of high fever and a shiny, bright red, raised plaque with sharply demarcated, elevated borders, often displaying a "peau d'orange" texture. It typically occurs on the face or lower extremities. First-line therapy for mild cases is oral penicillin VK or cephalexin; severe cases require intravenous cefazolin.

Cellulitis is a deeper infection of the deep dermis and subcutaneous fat, primarily caused by Streptococcus pyogenes and Staphylococcus aureus. It has a more gradual, indolent onset, presenting as a flat, warm, erythematous area with ill-defined, poorly demarcated borders. Non-purulent cellulitis is treated with cephalexin or dicloxacillin. Purulent cellulitis (suggestive of MRSA) requires trimethoprim-sulfamethoxazole, doxycycline, or clindamycin. Severe, systemic infections warrant admission for intravenous vancomycin. The leading edge of erythema should be marked to monitor treatment response.

Herpes Zoster (Shingles)

Herpes zoster is caused by the reactivation of latent Varicella-Zoster Virus (VZV) in the sensory dorsal root ganglia. It presents as a painful, vesicular eruption in a unilateral dermatomal distribution that does not cross the midline. A burning prodrome in the affected dermatome typically precedes the rash by 48 to 72 hours. The lesions progress from erythematous macules to grouped vesicles, which eventually crust and heal over 2 to 4 weeks.

Complications include:

Postherpetic Neuralgia (PHN): Pain persisting >3 months after rash resolution. Advanced age is the strongest risk factor. Manage with gabapentin, pregabalin, or amitriptyline.

Herpes Zoster Ophthalmicus (HZO): Reactivation in the ophthalmic division of the trigeminal nerve (CN V1). Vesicles on the tip of the nose (Hutchinson sign) indicate nasociliary branch involvement and predict ocular disease. An urgent ophthalmology consult is mandatory to evaluate for corneal keratitis and prevent permanent vision loss.

Ramsay Hunt Syndrome: Reactivation in the geniculate ganglion of the facial nerve (CN VII), presenting with facial nerve palsy, ear pain, and auditory canal vesicles.

Oral antivirals (valacyclovir 1 g three times daily, famciclovir, or acyclovir) should be initiated within 72 hours of rash onset to accelerate healing and reduce PHN. Immunocompromised patients, those with cranial nerve involvement, or patients with disseminated disease (crossing the midline or involving >3 dermatomes) require admission for intravenous acyclovir.

Skin Cancer Screening and Identification

Skin cancers are classified into melanoma and non-melanoma skin cancers (basal cell carcinoma and squamous cell carcinoma). Melanoma arises from malignant transformation of melanocytes. Triage is guided by the ABCDE criteria: Asymmetry, Border irregularity, Color variation, Diameter >= 6 mm, and Evolution. The "ugly duckling sign" (a lesion differing significantly from surrounding nevi) is highly sensitive. For suspected melanoma, perform a full-thickness excisional biopsy with 1 to 3 mm margins. Avoid superficial shave biopsies because they can transect the base, preventing accurate measurement of Breslow thickness, which is the single most critical prognostic indicator. Wide local excision margins are driven by Breslow thickness: in situ requires 0.5–1 cm margins; <= 1 mm requires 1 cm margins; and > 1 mm requires 1 to 2 cm margins along with a sentinel lymph node biopsy.

Basal Cell Carcinoma (BCC) is the most common skin cancer, presenting as a pearly, translucent papule with telangiectasias, rolled borders, and a central ulceration. Treatment is standard excision with 4 mm margins for low-risk sites, or Mohs micrographic surgery for high-risk locations (face, nose, ears, scalp) or aggressive subtypes (morpheaform).

Squamous Cell Carcinoma (SCC) arises from malignant keratinocytes and presents as a firm, erythematous, scaly plaque or nodule, often with central ulceration. Precursors are actinic keratoses, which present as gritty, sandpaper-like papules on sun-exposed skin. Risk factors include UV exposure, chronic wounds (Marjolin ulcer), and immunosuppression (highly aggressive in organ transplant recipients). Treatment is surgical excision with 4 to 6 mm margins, or Mohs surgery for high-risk lesions.