Dermatology Flashcards

A 45-year-old patient presents with a violaceous, reticulated, net-like vascular pattern on the skin of the lower extremities that worsens in cold temperatures but does not resolve with rewarming. She has a history of systemic lupus erythematosus and antiphospholipid antibody syndrome. Which diagnosis best explains this finding?

Answer: Livedo racemosa

Livedo racemosa is a fixed, irregular, broken-net pattern that does NOT resolve with rewarming, distinguishing it from physiologic livedo reticularis. It is strongly associated with antiphospholipid antibody syndrome and systemic vasculitis. Erythema ab igne results from chronic heat exposure and shows a similar pattern but with hyperpigmentation. Cutis marmorata telangiectatica congenita is a congenital condition presenting at birth.

A 62-year-old man with long-standing hidradenitis suppurativa (HS) stage III develops a non-healing, exophytic, verrucous ulcer in a chronic sinus tract of the groin. Biopsy shows well-differentiated squamous cell carcinoma. Which eponymous term describes this transformation in the context of chronic inflammatory sinus tracts?

Answer: Marjolin's ulcer

Marjolin's ulcer refers to squamous cell carcinoma arising in chronic scars, wounds, or sinus tracts — including those from hidradenitis suppurativa, osteomyelitis, or burn scars. It tends to be aggressive with higher metastatic potential. Queyrat's erythroplasia and Bowen's disease are in situ SCC variants in distinct anatomic locations. Leser-Trélat is an eruptive seborrheic keratosis paraneoplastic sign.

A 38-year-old woman presents with painful oral erosions, flaccid blisters on the trunk, and positive Nikolsky sign. Direct immunofluorescence (DIF) shows intercellular IgG and C3 in a 'fishnet' pattern throughout the epidermis. Anti-desmoglein (Dsg) ELISA shows elevated anti-Dsg3 but normal anti-Dsg1 titers. Which clinical variant does this profile indicate?

Answer: Pemphigus vulgaris, mucosal-dominant type

Anti-Dsg3 alone (without anti-Dsg1) corresponds to mucosal-dominant pemphigus vulgaris, since Dsg3 is predominantly expressed in mucosal and suprabasal epidermal layers. When both anti-Dsg1 and anti-Dsg3 are elevated, cutaneous involvement is also prominent (mucocutaneous type). Pemphigus foliaceus targets Dsg1 only, causing superficial blisters without mucosal involvement. Paraneoplastic pemphigus shows additional anti-plakin antibodies.

A 55-year-old man on long-term hydroxyurea for polycythemia vera develops hyperpigmented macules on the shins, leg ulcers, and dermatomyositis-like skin changes on the dorsal hands. Which mechanism best explains the hydroxyurea-induced dermatomyositis-like eruption?

Answer: Inhibition of ribonucleotide reductase causing oxidative DNA damage in keratinocytes

Hydroxyurea inhibits ribonucleotide reductase, impairing DNA synthesis and repair. Chronic use leads to accumulation of oxidative DNA damage preferentially in sun-exposed and mechanically stressed keratinocytes, resulting in the characteristic dermatomyositis-like poikilodermatous changes, hyperpigmentation, and leg ulcers. This is a direct cytotoxic mechanism — not a classic immune hypersensitivity reaction.

A 29-year-old woman presents with grouped, pruritic vesicles on elbows, knees, and buttocks. Skin biopsy shows neutrophilic microabscesses at dermal papillae tips. DIF shows granular IgA deposits at the dermal-epidermal junction. She has no GI symptoms. Which HLA allele is most strongly associated with her condition and its underlying systemic disease?

Answer: HLA-DR3 and HLA-DQ2

Dermatitis herpetiformis (DH) is the cutaneous manifestation of celiac disease (gluten-sensitive enteropathy). It is almost universally associated with HLA-DQ2 (encoded by DQA1*05 and DQB1*02, linked to HLA-DR3) — >90% of DH patients carry this allele. HLA-B27 is associated with spondyloarthropathies. HLA-DR4/DQ8 is the second celiac-associated haplotype but far less prevalent in DH. HLA-Cw6 is associated with psoriasis.

A 70-year-old immunocompetent man develops a rapidly enlarging, dome-shaped, flesh-colored nodule with a central keratin-filled crater on sun-exposed skin of the cheek over 6–8 weeks. Histology shows a symmetric, well-differentiated, crateriform squamous proliferation with glassy cytoplasm and prominent eosinophilic keratinization. He is anxious about prognosis. Which statement is MOST accurate regarding management?

Answer: Excision with histologic margin assessment is preferred because reliable histologic distinction from invasive SCC is impossible on small biopsies

Keratoacanthomas (KA) may spontaneously regress, but they cannot be reliably distinguished from well-differentiated invasive squamous cell carcinoma on incisional or shave biopsy alone because the crateriform architecture is required for diagnosis. On a small biopsy, histologic features overlap entirely with SCC. Excision (or Mohs for facial lesions) provides definitive diagnosis and treatment simultaneously. Observation risks missing an SCC. Imiquimod has limited evidence and is not standard of care.