Dermatology MCQ - Infiltrative and Neoplastic Disorders - Small plaque parapsoriasis

A 45-year-old man presents with a 2-year history of persistent, asymptomatic, salmon-pink, scaly patches on his flanks. The lesions are approximately 2-4 cm in diameter and have a distinctive elongated, "finger-like" shape running along the lines of skin cleavage. They are non-indurated and there is no associated atrophy or poikiloderma. A biopsy is performed and reveals a superficial, perivascular lymphocytic infiltrate with mild spongiosis and focal parakeratosis. There is no significant epidermotropism or lymphoid atypia. What is the most likely diagnosis and the recommended management?

A. Mycosis fungoides, patch stage; initiate treatment with narrowband UVB phototherapy and schedule regular follow-up.
B. Pityriasis rosea; provide reassurance that the condition will resolve spontaneously.
C. Small plaque parapsoriasis (digitate dermatosis); reassure the patient of its benign nature and consider treatment with emollients or low-potency topical corticosteroids for cosmetic purposes.
D. Nummular eczema; treat with a mid-potency topical corticosteroid.
E. Tinea corporis; perform a potassium hydroxide (KOH) preparation and treat with a topical antifungal if positive.

Correct Answer: C. Small plaque parapsoriasis (digitate dermatosis); reassure the patient of its benign nature and consider treatment with emollients or low-potency topical corticosteroids for cosmetic purposes.

Answer and Explanation

The correct answer is C. This question describes the pathognomonic presentation of small plaque parapsoriasis (SPP), also known as digitate dermatosis. The key clinical clues are the chronicity, the location on the trunk (especially the flanks), the small size of the plaques (<5 cm), and the characteristic "digitate" or "finger-like" morphology where the lesions align with skin cleavage lines. The benign histology showing only spongiosis and a non-atypical infiltrate supports this diagnosis. SPP is widely regarded as a benign condition with an excellent prognosis and minimal, if any, risk of progression to cutaneous T-cell lymphoma (CTCL).

Why the Other Options are Incorrect:

• A. Mycosis fungoides, patch stage: This is the most important condition to rule out. However, the digitate morphology, lack of significant epidermotropism or cytologic atypia on biopsy, and the long-standing, stable history are all strongly against a diagnosis of MF. Lesions of MF are typically more polymorphic, irregular in shape, and may show poikiloderma.

• B. Pityriasis rosea: While PR can have a "Christmas tree" distribution, it is an acute, self-limiting eruption that resolves within 6-8 weeks, not a persistent condition lasting for years.

• D. Nummular eczema: This presents as very pruritic, coin-shaped (nummular), eczematous plaques, often on the extremities. The lesions described are neither coin-shaped nor typically pruritic.

• E. Tinea corporis: This typically presents as an annular, scaly plaque with a raised, leading edge and central clearing. The digitate pattern is not characteristic of a fungal infection, and a KOH preparation would be negative.

Additional High-Yield Information for Exams:

• Clinical Features:

  • Morphology: The digitate pattern is the single most recognizable feature.

  • "Wrapper-like" Scale: A fine, "branny" or "cigarette-paper-like" scale that can be easily peeled off.

  • Lack of Symptoms: It is almost always asymptomatic, which helps distinguish it from eczematous conditions.

• Histopathology: The findings are often non-specific and can overlap with chronic dermatitis. They typically include:

  • Focal parakeratosis (retention of nuclei in the stratum corneum).

  • Mild spongiosis (intercellular edema in the epidermis).

  • A mild, superficial perivascular lymphocytic infiltrate.

  • The absence of significant epidermotropism and lymphoid atypia is crucial for distinguishing it from early mycosis fungoides.

• Differential Diagnosis: The main differential, as stated, is early mycosis fungoides. Key distinguishing features favor SPP:

  • SPP: Digitate morphology, long-term stability, benign histology.

  • MF: Irregular/variegated patches, history of progression, histology with atypia and epidermotropism.

• Prognosis: The prognosis for SPP is excellent. It is considered a benign inflammatory dermatosis. The risk of progression to CTCL is so low that it is considered negligible by most experts. The lesions may persist for decades but typically do not cause any medical problems.

• Management & Rationale:

  • Rationale: The primary goal is to correctly diagnose the condition to provide reassurance and avoid unnecessary, aggressive treatments or anxiety about lymphoma.

  • First-line: Reassurance and education about the benign nature of the condition. No treatment is medically necessary.

  • Treatment (if desired for cosmetic reasons):

    • Emollients to reduce scaling.

    • Low- to mid-potency topical corticosteroids can temporarily reduce the appearance of the lesions.

    • Natural sunlight or narrowband UVB phototherapy is very effective at clearing the eruption, but it almost always recurs after discontinuation of treatment.

  • Follow-up: Unlike large plaque parapsoriasis, routine, long-term surveillance for lymphoma is not standard practice for classic SPP. A single follow-up visit to confirm stability may be offered for reassurance, but patients do not require lifelong monitoring.