Random Dermatology MCQ - Mycosis fungoides

A 45-year-old man presents with a several-month history of a slowly enlarging, asymptomatic plaque on his lower back. Examination reveals a well-demarcated, erythematous to orange-pink plaque with a faint yellowish hue and a 'pasted-on' appearance. There is no scale. The most likely diagnosis is:

A. Tinea corporis
B. Plaque psoriasis
C. Mycosis fungoides
D. Necrobiosis lipoidica
E. Granuloma annulare

Correct Answer: C. Mycosis fungoides

Answer & Explanation

Explanation:
The presentation described is classic for the patch stage of mycosis fungoides (MF), the most common form of cutaneous T-cell lymphoma. Key diagnostic clues include:

• Chronic, slowly progressive course: Lesions persist and enlarge over months to years.

• Morphology: Well-demarcated, often irregularly shaped, erythematous patches or thin plaques.

• Color: Can range from pink to orange-pink or salmon-colored (often referred to as fawn-colored). The yellowish hue is a subtle but recognized feature.

• "Pasted-on" appearance: The lesions appear superficially attached to the underlying skin, as if they were smeared or pasted on.

• Lack of scale or mild scaling: Scale is often absent or fine, unlike the thick, silvery scale of psoriasis.

The other options are incorrect:

• A. Tinea corporis typically presents as an annular, scaly, and often pruritic plaque with a leading edge of scale and central clearing.

• B. Plaque psoriasis is characterized by well-demarcated, thick, erythematous plaques with adherent, silvery-white scale.

• D. Necrobiosis lipoidica presents as well-demarcated, waxy, yellow-brown plaques with telangiectasias, typically on the shins. The center often becomes atrophic.

• E. Granuloma annulare typically presents as annular, flesh-colored to erythematous papules and plaques, often with a raised, firm border and a central depression, without the orange-pink or "pasted-on" quality.

Key Associations for Mycosis Fungoides

• Histopathology: The hallmark is the presence of Pautrier microabscesses (collections of atypical lymphocytes within the epidermis). There is a band-like infiltrate of atypical T-lymphocytes in the upper dermis with epidermotropism (lymphocytes preferentially migrating into the epidermis). The lymphocytes are typically CD3+, CD4+, and CD45RO+ (memory T-cells), with a loss of CD7.

• Differential Diagnosis: Includes chronic spongiotic dermatitis (eczema), psoriasis, parapsoriasis (large plaque parapsoriasis is considered a precursor to MF), and drug eruptions.

• Prognosis: This is highly stage-dependent. Limited patch/plaque disease has an excellent prognosis with a life expectancy similar to age-matched controls. Tumor stage, erythroderma, and nodal or visceral involvement confer a worse prognosis.

• Management: For early-stage disease (patches/plaques), first-line treatments are skin-directed therapies such as topical corticosteroids, topical nitrogen mustard, topical carmustine, phototherapy (NB-UVB, PUVA), and local radiotherapy. Systemic therapies (e.g., interferon, retinoids, methotrexate, targeted agents) are reserved for more advanced or refractory disease.