ABD Dermatology cheat sheet

Epidermis renews via keratinocyte transit from basal layer to stratum corneum in roughly 28 days; retinoids shorten this turnover time.

Melanin is synthesized by tyrosinase converting tyrosine to DOPA then dopaquinone; eumelanin is brown-black, pheomelanin is red-yellow.

Type I collagen dominates the dermis; type VII collagen forms anchoring fibrils at the dermoepidermal junction and is defective in dystrophic epidermolysis bullosa.

Langerhans cells are CD1a-positive epidermal antigen-presenting cells; Merkel cells are cytokeratin-20-positive mechanoreceptors.

Eccrine glands are cholinergic and widespread; apocrine glands are adrenergic, open into hair follicles, and concentrate in axillae and anogenital skin.

Skin barrier function depends on corneocyte cornified envelopes, filaggrin, and intercellular lipid lamellae rich in ceramides, cholesterol, and free fatty acids.

Cutaneous immune surveillance uses CLA-positive T cells homing to skin via E-selectin and CCR4/CCR10 chemokine interactions.

Wound healing phases are hemostasis, inflammation, proliferation, and remodeling; myofibroblasts drive contraction and collagen III is replaced by collagen I.

Psoriasis: well-demarcated erythematous plaques with silvery scale on extensor surfaces; Auspitz sign and Koebner phenomenon are supportive findings.

Atopic dermatitis follows the itch-scratch cycle; flexural distribution in children and adults, and filaggrin loss-of-function mutations increase risk.

Contact dermatitis is diagnosed by patch testing; irritant reactions are dose-dependent and non-immunologic, allergic reactions are type IV delayed hypersensitivity.

Pemphigus vulgaris shows flaccid bullae and positive Nikolsky sign with intraepidermal IgG deposition; bullous pemphigoid shows tense bullae with linear IgG/C3 at the basement membrane.

Erythema multiforme target lesions favor acral sites and are commonly triggered by HSV; Stevens-Johnson syndrome/TEN involves mucosal and widespread epidermal detachment.

Acne vulgaris pathogenesis combines follicular hyperkeratinization, sebum excess, Cutibacterium acnes colonization, and inflammation.

Melanoma ABCDE criteria: Asymmetry, Border irregularity, Color variation, Diameter greater than 6 mm, Evolving lesion.

Urticaria: transient pruritic wheals lasting under 24 hours per lesion; angioedema involves deeper dermis and subcutaneous tissue.

Topical corticosteroid potency is ranked class I (super-high) to class VII (low); thin skin sites such as face and intertriginous areas absorb more and favor lower potency.

Methotrexate for psoriasis is dosed weekly, not daily; supplement folic acid and monitor liver function, CBC, and renal function.

Isotretinoin is teratogenic: require two negative pregnancy tests before initiation and monthly testing plus two contraceptive methods.

Biologics targeting TNF-alpha, IL-17, and IL-23 require pretreatment tuberculosis screening and hepatitis B evaluation.

Phototherapy with narrowband UVB (311-313 nm) is first-line for widespread psoriasis and vitiligo; PUVA adds oral or topical psoralen plus UVA.

Systemic glucocorticoid tapering avoids adrenal insufficiency; abrupt withdrawal can precipitate rebound disease or adrenal crisis.

Dapsone requires G6PD testing before use because of hemolytic anemia risk; methemoglobinemia is another adverse effect.

Patch test readings occur at 48 and 72-96 hours; delayed readings up to 7 days improve detection of late reactions.

Acantholysis is loss of intercellular adhesion; spongiosis is intercellular edema with widened spaces between keratinocytes.

Lichenoid dermatitis shows a band-like lymphocytic infiltrate obscuring the dermoepidermal junction with basal vacuolar change and Civatte bodies.

Pemphigus vulgaris: suprabasal acantholysis with tombstoning of basal keratinocytes and direct immunofluorescence showing intercellular IgG.

Bullous pemphigoid: subepidermal blister with eosinophils and linear IgG/C3 along the basement membrane on direct immunofluorescence.

Dermatofibroma shows a dermal proliferation of spindle cells with collagen trapping at the periphery and overlying epidermal induction.

Mycosis fungoides: epidermotropic atypical lymphocytes with Pautrier microabscesses and cerebriform nuclei; CD4-positive T-cell clone.

Basal cell carcinoma: nests of basaloid cells with peripheral palisading and clefting between tumor and stroma; mucin in stroma.

Squamous cell carcinoma in situ (Bowen disease) shows full-thickness epidermal atypia with intact basement membrane and loss of maturation.

Miliaria crystallina involves superficial duct obstruction with clear vesicles; miliaria rubra involves deeper obstruction with erythematous papules.

Erythema toxicum neonatorum shows eosinophil-rich pustules on erythematous base in the first days of life and resolves spontaneously.

Transient neonatal pustular melanosis presents at birth with pustules that rupture leaving collarettes of scale and hyperpigmented macules.

Hemangiomas of infancy proliferate in the first months then involute; complicated lesions may require beta-blockers such as oral propranolol.

Kawasaki disease: fever over 5 days plus conjunctivitis, mucosal changes, rash, extremity changes, and cervical lymphadenopathy; risk of coronary aneurysms.

Atopic dermatitis in infants favors cheeks and extensor surfaces; in older children flexural creases predominate.

Staphylococcal scalded skin syndrome results from exfoliative toxin causing superficial intraepidermal cleavage and is treated with antibiotics.

Albinism and piebaldism both cause congenital leukoderma; piebaldism shows a white forelock and ventral midline depigmented patches.

Cutaneous lupus: acute malar rash, subacute annular or psoriasiform lesions with anti-Ro/SSA, and chronic discoid plaques with scarring.

Dermatomyositis shows Gottron papules, heliotrope eyelid eruption, and shawl sign; check for underlying malignancy and interstitial lung disease.

Systemic sclerosis: Raynaud phenomenon, sclerodactyly, and nailfold capillary changes; anti-Scl-70 links to diffuse disease, anticentromere to limited.

Sarcoidosis: noncaseating granulomas; cutaneous findings include erythema nodosum, lupus pernio, and maculopapular lesions.

Sweet syndrome: tender erythematous plaques with fever and neutrophilia; histology shows dense neutrophilic dermal infiltrate without vasculitis.

Vasculitis: palpable purpura suggests leukocytoclastic vasculitis; direct immunofluorescence and systemic workup guide management.

Morphea is localized scleroderma without systemic involvement; lesions show indurated plaques with a lilac border in active phase.

Drug reaction with eosinophilia and systemic symptoms (DRESS): rash, eosinophilia, and organ involvement, often with reactivation of herpesviruses.