Dermatology MCQ - Infiltrative and Neoplastic Disorders - Pityriasis lichenoides

A 12-year-old boy presents with a widespread eruption of erythematous, scaly papules on his trunk and proximal extremities. The lesions are in various stages of evolution, with some showing a central hemorrhagic crust. The rash developed over several weeks, and new crops of lesions continue to appear. The child is otherwise well. A biopsy is most likely to reveal which of the following histopathological patterns?

A. A dense, band-like lymphocytic infiltrate with vacuolar interface changes and necrotic keratinocytes.
B. Superficial perivascular dermatitis with spongiosis and parakeratosis.
C. Psoriasiform hyperplasia with neutrophils in the stratum corneum.
D. A wedge-shaped, periappendageal lymphocytic infiltrate with epidermal involvement and erythrocyte extravasation.
E. Subepidermal blister with eosinophils.

Correct Answer: D. A wedge-shaped, periappendageal lymphocytic infiltrate with epidermal involvement and erythrocyte extravasation.

Answer and Explanation

The correct answer is D. This question describes the classic presentation of pityriasis lichenoides chronica (PLC), or potentially the more acute form (PLEVA), given the hemorrhagic crust. The key clinical clue is the polymorphous appearance of lesions in different stages. The histopathology of pityriasis lichenoides is distinctive, often described as a "lymphocytic vasculitis." It is characterized by a dense, superficial and deep perivascular lymphocytic infiltrate that can have a wedge-shaped configuration. Key findings include epidermal involvement (parakeratosis, scale crust, sometimes necrosis), erythrocyte extravasation (red blood cells outside vessels, accounting for the hemorrhagic appearance), and damage to the dermal blood vessels by lymphocytes.

Why the Other Options are Incorrect:

• A. A dense, band-like lymphocytic infiltrate... with necrotic keratinocytes: This is the classic histology of lichen planus or a lichenoid drug eruption. While interface changes and necrotic keratinocytes can be seen in pityriasis lichenoides, the infiltrate in lichen planus is typically more "band-like" and confined to the superficial dermis, and it lacks the significant erythrocyte extravasation and wedge-shaped pattern.

• B. Superficial perivascular dermatitis with spongiosis...: This describes a spongiotic dermatitis, such as eczema or pityriasis rosea. Pityriasis rosea can look similar clinically, but its histology is much less specific, showing focal spongiosis and a mild superficial perivascular lymphocytic infiltrate without the vasculitic and hemorrhagic features.

• C. Psoriasiform hyperplasia with neutrophils...: This is the hallmark of psoriasis. While PLC can have some psoriasiform hyperplasia, it lacks the regular elongation of rete ridges and the consistent presence of neutrophilic microabscesses.

• E. Subepidermal blister with eosinophils: This describes a bullous pemphigoid or other autoimmune blistering disease, which is not consistent with the clinical presentation.

Additional High-Yield Information for Exams:

• Spectrum of Disease:

  • Pityriasis Lichenoides et Varioliformis Acuta (PLEVA): More acute, with hemorrhagic, necrotic, and vesicular papules that may ulcerate. Histology shows a more intense infiltrate with greater keratinocyte necrosis.

  • Pityriasis Lichenoides Chronica (PLC): More chronic, with brownish-red, scaly papules that resolve slowly, often leaving post-inflammatory hypopigmentation or hyperpigmentation.

• Histopathology: The findings can be seen as a spectrum correlating with clinical acuity.

  • Parakeratosis and scale-crust.

  • Lymphocytic Infiltrate: Dense, perivascular, often "wedge-shaped," composed primarily of T-cells.

  • Erythrocyte Extravasation: A very important clue.

  • Interface Dermatitis: Vacuolar change along the dermo-epidermal junction.

  • Necrotic Keratinocytes: More prominent in PLEVA.

• Differential Diagnosis: The clinical differential is broad and includes:

  • Guttate Psoriasis: More uniform, scaly papules.

  • Lymphomatoid Papulosis: A crucial mimic and part of the differential. Lesions are often larger, more nodular, and histology shows large, atypical CD30+ lymphocytes. Pityriasis lichenoides can sometimes overlap with or evolve into lymphomatoid papulosis.

  • Viral Exanthem

  • Secondary Syphilis

• Prognosis: The course is variable and often self-limited, but it can persist for months to years. It is generally considered a benign, inflammatory condition, though there are rare reports of progression to cutaneous T-cell lymphoma.

• Management & Rationale:

  • Rationale: Treatment is aimed at controlling the outbreak and speeding resolution, as no therapy is universally curative.

  • First-line: Observation and emollients. Many cases resolve spontaneously.

  • First-line Medical Therapy: Oral Tetracyclines (e.g., doxycycline, minocycline) or Erythromycin are often used for their anti-inflammatory properties, especially in children.

  • Phototherapy: Narrowband UVB is a very effective second-line treatment.

  • Topical Therapies: Topical corticosteroids and tacrolimus can be used for symptomatic relief but are rarely curative.