High-Yield Dermatology for USMLE Step 3: Melanoma, Drug Reactions, and Skin Signs

Here's something most Step 3 prep materials get wrong about dermatology: they treat it like a visual pattern-matching game. Memorize the rash, pick the diagnosis, move on. But the questions you'll actually face — especially on CCS — aren't asking you to identify psoriasis from a photo. They're asking whether you can recognize that a patient on allopurinol who develops a fever and diffuse rash needs that drug stopped right now, not after you've ordered a skin biopsy. The skin is a window into everything else going on, and Step 3 tests whether you can read it under pressure.

Skin Malignancies

Melanoma

The ABCDE criteria (asymmetry, border irregularity, color variation, diameter > 6 mm, evolution) are drilled into every med student. They work. But I've seen residents freeze on the next step: any lesion that triggers clinical suspicion gets an excisional biopsy with narrow 1–2 mm margins. Not a shave biopsy. Not punch through the center. Excisional. You need architecture and depth — that's what drives every decision downstream.

Staging after biopsy:

• Breslow thickness (measured in mm) is the single most important prognostic factor and determines T-stage
• Sentinel lymph node biopsy (SLNB) is indicated for tumors > 1 mm thick, or > 0.8 mm if ulcerated
• A positive SLNB used to automatically trigger completion lymph node dissection, but this has shifted — observation plus adjuvant systemic therapy (checkpoint inhibitors or targeted therapy) is now standard at many centers after the DeCOG-SLT and MSLT-II trials showed no survival benefit from completion dissection

Metastatic melanoma treatment:

• Test every metastatic melanoma for BRAF V600E mutation (present in roughly 50% of cutaneous melanomas)
• BRAF-mutated: combination BRAF + MEK inhibition — dabrafenib 150 mg BID + trametinib 2 mg daily, or vemurafenib + cobimetinib. These produce rapid tumor shrinkage, which matters when you have a patient with symptomatic brain mets or organ compromise
• All metastatic melanoma (regardless of BRAF): PD-1 checkpoint inhibitors — pembrolizumab 200 mg IV q3 weeks or nivolumab 480 mg IV q4 weeks. The response rate is lower than targeted therapy, but responses are far more durable. In my experience tutoring residents, Step 3 loves testing the concept that checkpoint inhibitors work across BRAF subtypes while BRAF/MEK inhibitors do not

Contrarian point: The combination of nivolumab + ipilimumab (dual checkpoint blockade) has the highest response rate in metastatic melanoma — around 58% — but comes with a roughly 60% rate of grade 3–4 immune-related adverse events. Step 3 may present a scenario where you need to weigh aggressive combination immunotherapy against the toxicity profile. Don't reflexively pick the most aggressive option in a patient with significant comorbidities.

Clinical pearl: Acral lentiginous melanoma — on palms, soles, and under the nail plate — is the most common melanoma subtype in Black, Hispanic, and Asian patients. It's not caused by UV exposure and won't be in a sun-damaged distribution. I've watched test-takers miss this because they associate melanoma exclusively with fair skin and sun exposure. If you see a pigmented streak in a nail bed or a darkening patch on a sole, melanoma is on your differential regardless of skin tone.

Squamous Cell Carcinoma (SCC)

SCC presents as a hyperkeratotic, crusted papule or non-healing ulcer on sun-damaged skin. The classic patient is an older man with decades of occupational sun exposure or an immunosuppressed organ transplant recipient.

Key management points:

• Actinic keratoses (AKs) are precursors — treat with cryotherapy, topical 5-fluorouracil (Efudex 5% cream BID for 2–4 weeks), or topical imiquimod to prevent progression
• SCC has real metastatic potential, unlike BCC. Risk is highest on the lip, ear, and in immunocompromised patients
• Treatment: surgical excision with clear margins. Mohs micrographic surgery is first-line for high-risk locations (face, ears, lips, digits). Consider SLNB for high-risk, poorly differentiated tumors or those > 2 cm
• Cemiplimab (PD-1 inhibitor) is now approved for locally advanced or metastatic SCC not amenable to surgery — a surprisingly common board-relevant update that many prep courses haven't caught up with

Basal Cell Carcinoma (BCC)

BCC is the most common skin cancer. It almost never metastasizes — the danger is local tissue destruction.

Recognize it by the pearly, translucent papule with rolled borders, central ulceration (the classic "rodent ulcer"), and visible telangiectasias. The nose is the single most common site.

Treatment approach:

• Mohs surgery for high-risk locations (face, especially periocular, nasal, periauricular)
• Electrodesiccation and curettage (ED&C) for low-risk, superficial trunk and extremity lesions
• Topical imiquimod 5% cream (5 days/week for 6 weeks) or topical 5-FU for superficial BCC when surgery isn't preferred
• Vismodegib 150 mg daily (Hedgehog pathway inhibitor) for locally advanced or metastatic BCC — rare, but it does show up on exams

Severe Drug Reactions

Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN)

This is where Step 3 CCS cases can make or break your score. SJS/TEN is a dermatologic emergency, and the exam is testing one thing above all: did you stop the offending drug immediately?

I've run CCS review sessions where residents do everything right — order fluids, call ophthalmology, transfer to the burn unit — but lose points because they didn't discontinue the causative medication as their first action. The algorithm cares about sequence.

The drugs to know cold:

• Sulfonamides (TMP-SMX is the most common single culprit)
• Allopurinol (especially in the first 2 months of therapy)
• Anticonvulsants: carbamazepine, phenytoin, lamotrigine (lamotrigine risk is dose-dependent — this is why you titrate slowly)
• NSAIDs (piroxicam more than ibuprofen)
• Antibiotics: penicillins, fluoroquinolones

Classify by body surface area of epidermal detachment:

• SJS: < 10% BSA
• SJS/TEN overlap: 10–30% BSA
• TEN: > 30% BSA — this carries a mortality rate of 30–50%

Management sequence on CCS:

1. Stop the drug. Do this first. Before fluids. Before transfer.
2. Transfer to a burn unit or ICU — these patients lose fluid and thermoregulatory capacity like burn patients
3. Aggressive supportive care: IV fluid resuscitation, wound management with non-adherent dressings, nutritional support (often requires NG tube)
4. Ophthalmology consult — corneal involvement and symblepharon formation can cause permanent blindness. This is a frequently missed consult on CCS
5. IVIG (total dose 2–3 g/kg over 3–4 days) is used at many centers; cyclosporine 3 mg/kg/day is gaining ground as an alternative with some retrospective data suggesting mortality benefit
6. Systemic corticosteroids: controversial. Some evidence suggests early short-course steroids may help, but prolonged use increases infection risk in patients with massive epidermal loss. The safe exam answer is to avoid them

Surprising fact: HLA-B*5801 testing before starting allopurinol can virtually eliminate the risk of allopurinol-induced SJS/TEN in high-risk populations (Southeast Asian, African American, Korean descent). This pharmacogenomic screening is now recommended by the ACR before initiating allopurinol in these groups. Expect to see this tested.

Nikolsky sign — lateral pressure on normal-appearing skin causes the epidermis to shear off, forming a blister. It's positive in SJS/TEN and pemphigus vulgaris (but not bullous pemphigoid — that distinction is a favorite board question).

Drug Hypersensitivity Reactions by Type

Reaction	Mechanism	Classic Trigger	How to Recognize It
Urticaria / angioedema	IgE-mediated (type I)	Penicillin	Onset within minutes to 1 hour; hives, lip/tongue swelling
Maculopapular exanthem	T-cell mediated (type IV)	Amoxicillin (especially with concurrent EBV)	Most common drug rash; diffuse, blanching, often appears day 7–14
SJS/TEN	Cytotoxic T-cell (CD8+)	Sulfonamides, allopurinol, anticonvulsants	Mucosal involvement (≥2 sites), targetoid lesions, skin detachment
DRESS syndrome	T-cell mediated	Anticonvulsants (carbamazepine, phenytoin), allopurinol, dapsone	Onset 2–8 weeks after drug start; high fever, facial edema, eosinophilia, hepatitis or nephritis
Serum sickness-like	Immune complex (type III)	Cefaclor, infliximab, rituximab	Fever, urticaria, polyarthralgia; onset 7–21 days post-exposure

A distinction I hammer home to every resident: DRESS syndrome has a long latency period (2–8 weeks) and causes internal organ damage — the liver is most commonly affected. If you see eosinophilia + liver enzyme elevation + rash + fever weeks after starting a new anticonvulsant, that's DRESS until proven otherwise. It mimics infectious mononucleosis, and I've seen it missed for exactly that reason.

Skin Signs of Systemic Disease

These show up as quick-hit questions. The exam gives you a skin description and expects you to connect it to the underlying systemic condition — or vice versa. Don't just memorize the pairs; understand why the connection exists so you can reason through unfamiliar presentations.

Skin Finding	Systemic Disease	The Clinical Logic
Erythema nodosum — tender, red-purple nodules on the anterior shins	Sarcoidosis, IBD, streptococcal infection, coccidioidomycosis, drugs (OCPs, sulfonamides)	Septal panniculitis; represents a hypersensitivity response to the underlying trigger
Pyoderma gangrenosum — rapidly enlarging, painful ulcer with a violaceous undermined border	IBD (UC > Crohn's), RA, myeloproliferative disorders (AML, MDS)	Pathergy phenomenon: lesions worsen with surgical debridement — do NOT debride
Acanthosis nigricans — velvety, hyperpigmented plaques in the axillae and posterior neck	Insulin resistance / type 2 DM; if sudden-onset and severe → GI malignancy (gastric adenocarcinoma)	Driven by excess insulin or IGF-1 stimulating keratinocyte proliferation
Dermatitis herpetiformis — intensely pruritic grouped vesicles on elbows, knees, buttocks	Celiac disease (nearly 100% will have duodenal villous atrophy on biopsy)	Treat with dapsone 100 mg daily for rapid symptom relief + strict gluten-free diet for long-term control
Necrobiosis lipoidica — yellowish-brown, waxy, atrophic plaques on the shins	Diabetes mellitus (but most diabetics never develop it)	Collagen degeneration from microangiopathy
Xanthelasma / eruptive xanthomas	Hyperlipidemia (familial hypercholesterolemia, hypertriglyceridemia)	Lipid deposition in skin; eruptive xanthomas suggest triglycerides > 1000 mg/dL
Palpable purpura — non-blanching, raised lesions on the lower extremities	IgA vasculitis (Henoch-Schönlein), ANCA vasculitis, endocarditis, cryoglobulinemia	Small-vessel vasculitis with RBC extravasation; palpability distinguishes vasculitis from thrombocytopenia

A trap I see on practice exams: Pyoderma gangrenosum exhibits pathergy — it gets worse with surgical debridement. The instinct to clean up a necrotic-looking wound is strong, but operating on PG is the wrong answer. Treatment is immunosuppression: high-dose prednisone (0.5–1 mg/kg/day), cyclosporine, or TNF inhibitors.

Inflammatory Dermatoses

Psoriasis

Well-demarcated, salmon-pink plaques covered in silvery-white scale on the extensor surfaces (elbows, knees) and scalp. The Auspitz sign — pinpoint bleeding when you peel off the scale — is classic but rarely tested directly. What Step 3 actually cares about is the treatment ladder and the systemic associations.

Associations you need to know:

• Psoriatic arthritis — affects roughly 30% of psoriasis patients. Look for DIP joint involvement (unlike RA), dactylitis ("sausage digits"), enthesitis, and spondylitis. Can be seronegative (RF-negative, anti-CCP negative)
• Metabolic syndrome — patients with severe psoriasis have elevated cardiovascular risk independent of traditional risk factors. This is increasingly tested
• IBD — Crohn's disease in particular

Treatment ladder (know this progression):

• Mild: topical high-potency corticosteroids (betamethasone dipropionate 0.05%, clobetasol 0.05%) + vitamin D analogs (calcipotriene). Use lower-potency steroids on the face and intertriginous areas
• Moderate: narrowband UVB phototherapy (311 nm) — the workhorse of moderate psoriasis treatment
• Moderate-severe: methotrexate 15–25 mg weekly (monitor LFTs, CBC; teratogenic — pregnancy test required), cyclosporine (short-term use due to nephrotoxicity and hypertension), acitretin (retinoid — teratogenic for 3 years after discontinuation in women)
• Severe / refractory: biologics — TNF-α inhibitors (adalimumab, etanercept), IL-17 inhibitors (secukinumab, ixekizumab), IL-23 inhibitors (guselkumab, risankizumab). IL-17 and IL-23 inhibitors have largely overtaken TNF inhibitors for psoriasis based on superior PASI response rates

Atopic Dermatitis (Eczema)

Pruritic, erythematous, often lichenified patches in the flexural areas — antecubital fossae, popliteal fossae, neck, and wrists. Onset is typically in childhood, and it tends to improve with age (though not always).

Part of the atopic triad: eczema, allergic rhinitis, asthma. If a question gives you a child with eczema, expect a follow-up about asthma risk.

Treatment approach:

• Foundation: liberal emollients (ceramide-based creams applied within 3 minutes of bathing — the "soak and seal" technique), fragrance-free products, lukewarm baths
• Flares: topical corticosteroids — triamcinolone 0.1% ointment for the body, hydrocortisone 2.5% for the face. Ointments > creams for eczema (better barrier, fewer preservatives and sensitizers)
• Steroid-sparing: tacrolimus 0.1% ointment or pimecrolimus 1% cream — calcineurin inhibitors that don't cause skin atrophy. First-line for face and eyelid eczema
• Moderate-severe (systemic): dupilumab 300 mg SC every 2 weeks (IL-4/IL-13 monoclonal antibody) — this has genuinely transformed severe eczema management. Side effect to know: conjunctivitis in ~10% of patients, which is paradoxical since dupilumab treats allergic disease
• New option: JAK inhibitors (upadacitinib 15–30 mg daily, abrocitinib) for adults with moderate-severe disease — oral alternatives to dupilumab, but carry a boxed warning for cardiovascular events and malignancy based on class-wide JAK inhibitor data

One thing I wish someone had told me during Step 3 prep: eczema herpeticum — widespread HSV superinfection of eczematous skin — is a dermatologic emergency. The patient looks like their eczema flared, but you see punched-out erosions and vesicles in a monomorphic pattern. Start IV acyclovir 10 mg/kg q8h immediately. Don't wait for cultures. This shows up on CCS and the exam penalizes delays in antiviral initiation.

FAQs

Q: How does Step 3 test dermatology differently from Step 2? Step 2 focuses on diagnosis — here's a rash, name it. Step 3 shifts to management decisions and complications. You'll get a patient already diagnosed with SJS and need to manage them on CCS, or a melanoma patient where you need to choose between immunotherapy and targeted therapy based on mutation status. The emphasis is on what you do, not what you recognize.

Q: Do I actually need to memorize the Breslow thickness cutoffs for melanoma staging? Know that > 1 mm (or > 0.8 mm if ulcerated) triggers SLNB. You don't need to recite every T-stage cutoff. What matters more is the decision tree: biopsy → Breslow depth → SLNB yes/no → systemic therapy selection based on BRAF status and disease extent.

Q: What's the single highest-yield drug reaction to know for Step 3? SJS/TEN caused by sulfonamides or allopurinol. It tests drug identification, emergency management, CCS sequencing, and the concept of SCORTEN prognostic scoring all in one case. If you only study one drug reaction deeply, make it this one.

Q: Should I memorize every skin-systemic disease association in the table above? Yes, but strategically. Erythema nodosum, pyoderma gangrenosum, acanthosis nigricans, and dermatitis herpetiformis are the four highest-yield. Each one has appeared on recent Step 3 forms. The others are worth knowing but less frequently tested as standalone questions.

Q: How do I approach a CCS case where a rash develops in a hospitalized patient on multiple medications? Look at timing. Drug rash within hours of a new drug → think IgE-mediated urticaria. Rash at 7–14 days → maculopapular drug eruption. Rash at 2–8 weeks with fever and eosinophilia → DRESS. Mucosal involvement with skin detachment → SJS/TEN. Stop the most likely culprit first, then manage based on severity. On CCS, always stop the drug before ordering the workup.

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Practice Dermatology Questions

Want to put this knowledge to work? Step3Sim has free USMLE Step 3 practice questions covering dermatology and every other organ system — including CCS-style cases for SJS/TEN and melanoma management that mirror exactly what you'll face on exam day. The best way to lock in these associations is to get them wrong in practice before you get them right on the real thing.