A 62-year-old man presents with a 6-year history of pruritic, slowly progressive red patches and plaques on the buttocks, lower abdomen, inner thighs, and flanks. The lesions show characteristic wrinkled, cigarette-paper atrophy and have been resistant to multiple courses of topical corticosteroids. Examination reveals asymmetric, fixed erythematous to violaceous patches with some infiltrated plaques. Notably, the lesions spare the face, dorsal hands, and distal forearms. Skin biopsy confirms mycosis fungoides with epidermotropic T-cell infiltrate and clonal T-cell receptor gene rearrangement. The distribution pattern marked **B** in the diagram is a key diagnostic clue. Which of the following best explains the clinical significance of this distribution pattern in mycosis fungoides?
A. The pattern is pathognomonic for erythrodermic mycosis fungoides and predicts transformation to Sézary syndrome
B. Bathing-trunk and sun-protected areas are preferentially involved because malignant T cells are photosensitive and avoid UV-exposed skin
C. Lesions in bathing-trunk distribution indicate systemic dissemination and warrant immediate systemic chemotherapy
D. The distribution reflects the anatomical predilection of cutaneous T-cell lymphoma for intertriginous and occluded areas with higher skin temperature and humidity, sparing sun-exposed sites
Explanation
Why option 1 is correct
The bathing-trunk and sun-protected distribution is a hallmark diagnostic clue in mycosis fungoides. The malignant T-cell clone preferentially involves areas of the skin that are chronically covered and protected from sunlight—the buttocks, lower abdomen, inner thighs, and flanks—while sparing sun-exposed sites (face, dorsal hands, distal forearms). This distribution pattern reflects the biology of cutaneous T-cell lymphoma, which arises in and is maintained within the skin microenvironment of occluded, warm, humid areas. The sparing of sun-exposed areas is a key diagnostic feature that distinguishes mycosis fungoides from other inflammatory dermatoses and is explicitly recognized in WHO-EORTC classification criteria as a suggestive distributional pattern in early-stage disease (Willemze R. WHO-EORTC classification of cutaneous lymphomas 2023 update).
Why each distractor is wrong
Option 2: While intertriginous areas may be involved, the correct answer specifically identifies the sun-protected nature of the distribution as the key pathophysiologic principle. The question stem emphasizes that the distribution "spares sun-exposed sites," making photosensitivity of malignant cells the more precise mechanistic explanation than general temperature/humidity preferences.
Option 3: Bathing-trunk distribution in mycosis fungoides does NOT indicate systemic dissemination. This patient has stage IB disease (patches and plaques >10% BSA with no nodal or blood involvement) and has a normal CBC with normal Sézary cell count. Systemic therapy is reserved for refractory or advanced disease, not for localized bathing-trunk pattern alone.
Option 4: Bathing-trunk distribution is characteristic of early-stage patch-plaque mycosis fungoides, NOT erythrodermic disease. Erythroderma (>80% BSA involvement) and Sézary syndrome (erythroderma + circulating atypical T cells) are advanced presentations. This patient has <10% BSA involvement and no erythroderma.
High-YieldNEET PG
Bathing-trunk and sun-protected distribution is a diagnostic hallmark of early mycosis fungoides; sparing of sun-exposed areas is a key clue that distinguishes it from inflammatory mimics.
Willemze R. WHO-EORTC classification of cutaneous lymphomas 2023 update. NCCN MF/SS guidelines 2024.
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