Dermatology MCQ - Infiltrative and Neoplastic Disorders - Parapsoriasis

A 55-year-old man presents with a several-month history of persistent, asymptomatic, pink-to-yellow, finely scaly patches on his trunk and proximal extremities. The patches are irregular in shape and vary in size from 2 to 10 cm. There is no significant induration, atrophy, or poikiloderma. A biopsy is performed and reveals a superficial, band-like lymphocytic infiltrate with mild epidermotropism of lymphocytes, but no significant cytologic atypia. Which of the following is the most accurate diagnosis and the most critical aspect of long-term management?

A. Large plaque parapsoriasis; this condition is considered a precursor to mycosis fungoides and requires lifelong, regular skin surveillance.
B. Pityriasis rosea; this is a self-limited condition that requires only reassurance and symptomatic treatment.
C. Small plaque parapsoriasis (digitate dermatosis); this is a benign condition with an excellent prognosis and requires no specific follow-up.
D. Nummular eczema; treatment with mid-potency topical corticosteroids is first-line.
E. Psoriasis vulgaris; treatment with topical vitamin D analogs and corticosteroids is indicated.

Correct Answer: A. Large plaque parapsoriasis; this condition is considered a precursor to mycosis fungoides and requires lifelong, regular skin surveillance.

Answer and Explanation

The correct answer is A. This question describes the classic presentation of large plaque parapsoriasis (LPP). The key clues are the persistent, large (>5 cm), irregular, scaly patches in an older adult and the histology showing a lymphocytic infiltrate with mild epidermotropism (lymphocytes entering the epidermis) but no overt atypia. This histological picture is often termed "lymphoid atypia" or "pre-mycotic." LPP is widely regarded as the most common precursor to mycosis fungoides (MF), the most common type of cutaneous T-cell lymphoma. Therefore, the cornerstone of management is vigilant, long-term monitoring for the development of frank lymphoma.

Why the Other Options are Incorrect:

• B. Pityriasis rosea: This is an acute, self-limiting eruption that typically presents with a "herald patch" and a Christmas-tree distribution on the trunk. It resolves within 6-8 weeks, unlike the persistent patches described here.

• C. Small plaque parapsoriasis (digitate dermatosis): This is characterized by small, round or finger-like (digitate) patches, typically on the flanks. It is generally considered a benign condition with a very low, if any, risk of progressing to lymphoma. The lesions described are large and irregular, which is not consistent with the digitate variant.

• D. Nummular eczema: This presents as very pruritic, coin-shaped (nummular), eczematous plaques, often on the extremities. The histology would show spongiosis (epidermal edema), not a band-like lymphocytic infiltrate with epidermotropism.

• E. Psoriasis vulgaris: Psoriasis presents with well-demarcated, thick, silvery-scaled plaques, often on the elbows, knees, and scalp. The histology is distinct, showing regular psoriasiform hyperplasia, parakeratosis with neutrophils, and dilated blood vessels.

Additional High-Yield Information for Exams:

• Classification and Terminology: The term "parapsoriasis" is historical and encompasses two main groups:

  1. Small Plaque Parapsoriasis (SPP): Benign, digitate lesions. Prognosis is excellent.

  2. Large Plaque Parapsoriasis (LPP): Considered a pre-malignant condition. When LPP shows features of poikiloderma (atrophy, telangiectasia, and mottled pigmentation), it is called poikiloderma vasculare atrophicans, which has an even stronger association with MF.

• Histopathology: The histology of LPP is often subtle and can be indistinguishable from early MF. Findings include:

  • A superficial, band-like (lichenoid) or perivascular lymphocytic infiltrate.

  • Epidermotropism: Lymphocytes within the epidermis, often lining up in the basal layer.

  • Lymphocytic Atypia: The lymphocytes may be slightly enlarged or hyperchromatic, but not overtly malignant.

  • The absence of Pautrier microabscesses (collections of atypical lymphocytes in the epidermis) helps distinguish it from definitive MF.

• Differential Diagnosis: The main differential is early-stage Mycosis Fungoides. This distinction can be incredibly difficult and sometimes requires multiple biopsies and T-cell receptor gene rearrangement studies to identify a clonal T-cell population.

• Prognosis: The risk of progression from LPP to MF is significant, with estimates ranging from 10% to 30% over a patient's lifetime. Progression can take years or even decades.

• Management & Rationale:

  • Rationale: The primary goal is early detection of transformation to MF. Treatment is also aimed at controlling the extent of the disease and potentially delaying progression.

  • First-line: Lifelong, regular (e.g., every 6-12 months) total-body skin examinations. Baseline photography can be invaluable for tracking changes.

  • First-line Treatment: Narrowband UVB (NB-UVB) phototherapy is often the treatment of choice. It is highly effective at clearing the lesions and may have a disease-modifying effect.

  • Other Treatments: Topical corticosteroids, topical nitrogen mustard, and topical bexarotene can be used.

  • Biopsy: Any patch that thickens, becomes indurated, develops tumors, or changes significantly should be re-biopsied immediately.