Dermatology MCQ - Infiltrative and Neoplastic Disorders - Large plaque parapsoriasis

A 60-year-old man presents with a several-year history of persistent, fixed, thin, erythematous patches on his buttocks and thighs. The patches are large, ranging from 8 to 15 cm in diameter, and have a "wrinkled paper" or atrophic appearance with subtle telangiectasias. They are mildly pruritic. A biopsy reveals a superficial band-like lymphocytic infiltrate with lymphocytes lining up along the dermo-epidermal junction and scattered into the epidermis (epidermotropism). The lymphocytes are slightly enlarged but without frank atypia. Which of the following is the most accurate diagnosis and the most appropriate long-term management strategy?

A. Mycosis fungoides; refer to oncology for staging and initiate skin-directed therapy.
B. Large plaque parapsoriasis; initiate narrowband UVB phototherapy and schedule regular, lifelong skin surveillance.
C. Chronic contact dermatitis; recommend patch testing and strict allergen avoidance.
D. Poikiloderma vasculare atrophicans; manage as a benign variant of photodamage with sun protection.
E. Tinea corporis; treat with a prolonged course of systemic antifungals.

Correct Answer: B. Large plaque parapsoriasis; initiate narrowband UVB phototherapy and schedule regular, lifelong skin surveillance.

Answer and Explanation

The correct answer is B. This question describes the classic presentation of large plaque parapsoriasis (LPP). The key clues are the large size of the patches (>5-10 cm), their fixed and persistent nature, and the early signs of poikiloderma (atrophy, telangiectasias, and mottled pigmentation described as a "wrinkled paper" appearance). The histology is highly suggestive of a pre-mycotic condition, showing epidermotropism without definitive atypia. LPP is considered the most common precursor to mycosis fungoides (MF), a cutaneous T-cell lymphoma. Therefore, management involves both treatment to control the lesions and vigilant, lifelong monitoring for malignant transformation.

Why the Other Options are Incorrect:

• A. Mycosis fungoides: This is the most critical differential. While the histology is suspicious, the description lacks definitive features of MF, such as significant lymphoid atypia or Pautrier microabscesses. Many experts consider LPP and early MF to exist on a spectrum, and a diagnosis of LPP implies a high risk of progression to MF. However, in the absence of definitive atypia, the most precise diagnosis at this stage is LPP.

• C. Chronic contact dermatitis: This is typically more pruritic and does not present with large, fixed, atrophic patches. The distribution on the buttocks and thighs is also not classic for contact dermatitis.

• D. Poikiloderma vasculare atrophicans: This is not a separate diagnosis but rather a descriptive term for the clinical appearance of the skin in this context. When LPP exhibits poikiloderma, it is called LPP with poikiloderma and carries the same, if not a higher, risk of progression to MF. It is not a benign variant of photodamage.

• E. Tinea corporis: This presents as annular, scaly, expanding plaques with a raised border, not large, atrophic, fixed patches. A potassium hydroxide (KOH) preparation would be negative.

Additional High-Yield Information for Exams:

• Clinical Features:

  • Location: Typically on sun-shielded areas like the buttocks, breasts, and flexures. This is a key distinguishing feature from photodamage.

  • Size: Plaques are by definition large (>5 cm, often >10 cm).

  • Surface: May be finely scaly and often show early atrophy and telangiectasias (poikiloderma).

• Histopathology: The findings are often on a spectrum between chronic inflammation and overt lymphoma ("lymphoid atypia").

  • Superficial, band-like (lichenoid) lymphocytic infiltrate.

  • Epidermotropism of lymphocytes without spongiosis.

  • The lymphocytes may be slightly enlarged (hyperchromatic).

  • The absence of Pautrier microabscesses (collections of atypical lymphocytes in the epidermis) and significant cytologic atypia helps distinguish it from definitive MF, but the line is often blurry.

• Differential Diagnosis: The main differential is early-stage Mycosis Fungoides. The distinction can be difficult and may require multiple biopsies and T-cell receptor gene rearrangement studies to detect 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 be slow, occurring over years or decades.

• Management & Rationale:

  • Rationale: The primary goals are to control the disease, improve symptoms, and enable early detection of transformation to MF.

  • First-line Treatment: Narrowband UVB (NB-UVB) phototherapy is highly effective. It clears lesions and may have a disease-modifying effect by modulating the local immune environment.

  • Other Skin-Directed Therapies: Topical corticosteroids, topical nitrogen mustard, and topical bexarotene.

  • Crucial Long-term Management: Lifelong, regular (e.g., every 6-12 months) total-body skin examinations. Any change in the lesions (thickening, ulceration, nodule formation) warrants an immediate re-biopsy.