| Psoriasis | Erythematous, well-marginated papules and plaques with silvery scale | Trunk, extensor surfaces, scalp | Auspitz sign, Koebner phenomenon, nail pitting | Hereditary predilection; onset in early 20s |
| Seborrheic dermatitis | Greasy, yellow scales | Mid chest, suprapubic, scalp, facial creases | Can overlap with psoriasis, “sebopsoriasis” | Debilitated, elderly, or infants (cradle cap) |
| Pityriasis rosea | Oval scaly macules with collarettes of scale following skin tension lines | Trunk, in Christmas tree pattern | Herald patch 1–2 wk before general eruption | Spring and fall, age 15–40 y old; viral exanthem, herpes 6 and 7 |
| Tinea corporis | Sharply demarcated, erythematous, scaly annular plaques; may coalesce into gyrate patterns | Trunk, legs, arm, neck | May need KOH/culture to diagnose; septate branching hyphae on KOH | All ages; from pets, soil, or autoinoculation from hands/feet; incubation days or months |
| Pityriasis (tinea) versicolor | Versicolored—red, salmon, light brown, dark brown, hypopigmented; well-demarcated scaly patches | Central upper chest and back | “Spaghetti and meatballs” on KOH; nonseptate pseudohyphae and yeast cells | Young adults, summer, hot humid environments |
| Atopic dermatitis | Erythematous scaly macules/patches; chronic with lichenification | Flexures > trunk | Spares the nose | Pruritic “itch that rashes”; atopic individuals |
| Contact dermatitis | Ill-defined vesicles forming plaques with scale; chronic lesions lichenified | Varies based on site of exposure; dorsal hands, face commonly affected | Burning > itching in irritant contact (vs. allergic contact) | Common allergens: nickel, poison ivy, fragrances Common irritants: soap, wet work/frequent hand washing, production work materials |
| Lichen planus | 5 Ps: purple, pruritic, polygonal, planar papules | Any skin, mucous membranes, hair follicles | Wickham striae, Koebner phenomenon | Age 20–60 y old |
| Secondary syphilis | At 2–10 wk, macular erythema on trunk, abdomen, inner extremities; followed by papular or papulosquamous lesions | Palms, soles, trunk | Serology | Great masquerader—can take any form; can be confused with pityriasis rosea |
| Scabies | Pruritic papules and burrows with crusting | Finger webs, wrists, axillae, areolae, umbilicus, abdomen, waistband, genitals | Scrapings show mites, feces, eggs | Can be chronic “7-year itch”; intensely pruritic, especially at night |