A 58-year-old man presents with a 3-year history of non-pruritic, finely scaling erythematous patches on his buttocks and lower trunk that have been refractory to topical corticosteroids. Punch biopsy shows epidermotropic atypical lymphocytes with cerebriform nuclei and the histologic finding marked **A** in the diagram. Immunohistochemistry reveals CD2+, CD3+, CD4+ tumor cells with loss of CD7. TCR gene rearrangement studies show a monoclonal T-cell population. What is the most likely diagnosis?
A. Cutaneous lupus erythematosus
B. Pityriasis rosea
C. Mycosis fungoides, patch stage
D. Psoriasis vulgaris
Explanation
Why Mycosis fungoides, patch stage is right
Mycosis fungoides (MF) is the most common cutaneous T-cell lymphoma (CTCL), an indolent non-Hodgkin lymphoma derived from mature skin-homing CD4+ memory T cells. The clinical presentation—non-pruritic erythematous patches on sun-protected areas (bathing-trunk distribution: buttocks, lower trunk) in a 50–60-year-old male—is classic for patch-stage MF. The histopathologic hallmark is epidermotropism (atypical lymphocytes infiltrating the epidermis without significant spongiosis), and the structure marked A (Pautrier microabscesses—small, well-circumscribed intraepidermal clusters of atypical lymphocytes) is highly suggestive of MF, though seen in only 10–37% of cases. The immunophenotype (CD2+, CD3+, CD4+, CD5+, loss of CD7, CD8 negative) and monoclonal TCR gene rearrangement confirm the diagnosis. Early MF is often misdiagnosed as eczema or psoriasis for years, making the distinction critical (WHO-EORTC Classification of Cutaneous Lymphomas 2018).
Why each distractor is wrong
Cutaneous lupus erythematosus: Presents with photosensitive plaques and systemic features; immunohistochemistry shows interface dermatitis with IgG/IgM deposition at the dermal-epidermal junction, not epidermotropic CD4+ T-cell infiltrates or Pautrier microabscesses. TCR gene rearrangement is polyclonal, not monoclonal.
Psoriasis vulgaris: Characterized by acanthosis, parakeratosis, and regular elongation of rete ridges with prominent spongiosis and neutrophilic microabscesses in the stratum corneum (not intraepidermal clusters of atypical lymphocytes). Immunohistochemistry shows polyclonal T-cell infiltrate, not monoclonal CD4+ population with loss of CD7.
Pityriasis rosea: A benign, self-limited exanthem with superficial perivascular lymphocytic infiltrate and spongiosis, lacking epidermotropism, Pautrier microabscesses, cerebriform nuclei, or monoclonal TCR rearrangement.
High-YieldNEET PG
Pautrier microabscesses (intraepidermal clusters of atypical lymphocytes) + epidermotropism + CD4+ phenotype + monoclonal TCR = mycosis fungoides; early patch-stage MF mimics eczema/psoriasis for years, so maintain high clinical suspicion in refractory dermatitis.
WHO-EORTC Classification of Cutaneous Lymphomas 2018
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