Dermatology MCQ - Inflammatory Dermatoses - Pityriasis Rubra Pilaris

A 60-year-old man presents with a widespread, orange-red, scaly eruption that has progressed over several months. On examination, you note large, scaly plaques with distinct islands of unaffected skin ("skip areas"), palmar-plantar keratoderma, and follicular hyperkeratotic papules on the dorsal fingers and knees. Which of the following is the most likely diagnosis and its characteristic histopathologic finding?

A) Pityriasis rubra pilaris (PRP); alternating vertical and horizontal ortho- and parakeratosis
B) Psoriasis vulgaris; regular acanthosis with suprapapillary thinning and Munro microabscesses
C) Mycosis fungoides; Pautrier microabscesses
D) Sézary syndrome; cerebriform nuclei in peripheral blood
E) Pityriasis rosea; focal parakeratosis and perivascular lymphocytic infiltrate

Correct Answer: A) Pityriasis rubra pilaris (PRP); alternating vertical and horizontal ortho- and parakeratosis

Explanation

This presentation is classic for the classic adult type (Type I) of Pityriasis Rubra Pilaris (PRP), a rare, idiopathic papulosquamous disorder.

Key Clinical Features of PRP:

• Erythroderma: Widespread, orange-red or salmon-colored erythema.

• "Skip Areas": Patches of uninvolved skin within the erythematous areas. This is a highly characteristic feature that helps distinguish it from psoriasis.

• Palmoplantar Keratoderma: Often a striking, waxy, orange-yellow thickening of the palms and soles.

• Follicular Hyperkeratosis: Keratotic, spiny papules centered on hair follicles, typically on the dorsal fingers, wrists, elbows, and knees.

Histopathologic Findings of PRP:

The histology is often more suggestive than diagnostic, but a characteristic feature is:

• Alternating vertical and horizontal orthokeratosis and parakeratosis: This is described as a "checkerboard" pattern of cornification. It is not always present but is highly supportive when seen.

• Other features include: mild to moderate acanthosis, a sparse lymphocytic perivascular infiltrate, and follicular plugging.

Why Not the Other Options?

• (B) Psoriasis vulgaris: Presents with well-demarcated, silvery-scaled plaques but lacks the orange hue, skip areas, and prominent follicular papules. Histology shows regular acanthosis, suprapapillary thinning, dilated capillaries, and Munro microabscesses (collections of neutrophils in the stratum corneum).

• (C) Mycosis fungoides (MF): The most common cutaneous T-cell lymphoma. Can present with patches, plaques, or tumors. Histology is diagnostic, showing Pautrier microabscesses (collections of atypical lymphocytes in the epidermis) and lymphocytes with cerebriform nuclei.

• (D) Sézary syndrome: The leukemic form of cutaneous T-cell lymphoma, characterized by erythroderma, lymphadenopathy, and Sézary cells (lymphocytes with cerebriform nuclei) in the peripheral blood.

• (E) Pityriasis rosea: A self-limiting eruption characterized by a herald patch followed by a generalized, Christmas-tree distribution of oval, scaly patches. Histology is non-specific, showing focal parakeratosis and a perivascular lymphocytic infiltrate.

Management of PRP:

• First-line: Oral retinoids (e.g., acitretin) are the treatment of choice.

• Second-line: Methotrexate or biologics (e.g., TNF-alpha inhibitors, Ustekinumab).

Prognosis:
The classic adult type (Type I) has a good prognosis, with many cases resolving spontaneously within 3-5 years.

Note: PRP is a great clinical mimicker of psoriasis. The salmon color, skip areas, and follicular papules are the key distinguishing clinical features. The "checkerboard" pattern of parakeratosis on histology is a helpful clue.