How dermatology is tested on Step 1
Sun-exposed lesions in older adults usually mean a skin cancer question.
| Lesion | Appearance | Key pathology |
|---|---|---|
| Basal cell carcinoma | Pearly papule, rolled edges, telangiectasias; upper face | Palisading basal cells; hedgehog pathway (PTCH1) mutations |
| Squamous cell carcinoma | Scaly or ulcerated nodule; lower lip, sun-exposed skin | Keratin pearls; risk ↑ after organ transplant |
| Melanoma | Asymmetric, irregular border, varied color, > 6 mm, evolving | S-100, BRAF V600E; Breslow depth |
| Actinic keratosis | Rough, scaly papule on sun-exposed skin | Precursor of squamous cell carcinoma |
| Seborrheic keratosis | Benign, "stuck-on," waxy plaque | Sudden eruption of many = Leser-Trélat sign (GI cancer) |
BRAF V600E melanoma can be treated with BRAF inhibitors such as vemurafenib.
Blistering diseases
The level of the split determines how the blister looks.
Inflammatory rashes and drug reactions
Common rashes have distinctive locations and histology.
- Psoriasis: well-defined silvery plaques on extensor surfaces and scalp; nail pitting; thickened epidermis with parakeratosis and Munro microabscesses; pinpoint bleeding when scale is removed (Auspitz sign).
- Atopic dermatitis (eczema): itchy rash on flexural surfaces in older children and adults, the face in infants; linked to asthma and allergic rhinitis.
- Allergic contact dermatitis: type IV reaction to nickel, poison ivy, or fragrances, shaped like the exposure.
- Urticaria: itchy wheals from mast cell degranulation (type I).
- Acanthosis nigricans: velvety dark skin in neck and axillary folds; insulin resistance or, rarely, gastric cancer.
Skin signs of systemic disease
Some rashes point to internal problems.
- Erythema nodosum: tender red nodules on the shins; sarcoidosis, IBD, streptococcal infection, coccidioidomycosis.
- Pyoderma gangrenosum: painful ulcer with a purple edge; IBD.
- Dermatomyositis: heliotrope rash and Gottron papules; screen for cancer.
- Vitiligo vs albinism: vitiligo is autoimmune loss of melanocytes; albinism has normal melanocyte numbers but defective melanin (tyrosinase).
Worked example
Use blister depth and age.
functions Worked example — blisters in an older adult
An 80-year-old has itchy, tense blisters on the thighs and abdomen. The mouth is spared. Rubbing nearby skin does not cause it to slough. Immunofluorescence shows a linear band at the basement membrane. What is the diagnosis?
- Tense blisters mean the roof is the full epidermis, so the split is below it.
- A negative Nikolsky sign and sparing of the mouth fit a subepidermal process.
- A linear deposit at the basement membrane means antibodies against hemidesmosomes.
- This is bullous pemphigoid, treated with corticosteroids.
Answer. Bullous pemphigoid.