Dermatology MCQ - Viral Infections - Pityriasis Rosea

A 19-year-old woman presents with a single, oval, erythematous, scaly plaque on her abdomen that appeared one week ago. She now has developed numerous smaller, similar lesions on her trunk, oriented along the skin cleavage lines. The rash is mildly pruritic. She is otherwise well and reports no recent drug exposures. Which of the following is the most likely diagnosis and its characteristic antecedent finding?

A) Pityriasis rosea; herald patch
B) Secondary syphilis; condylomata lata
C) Guttate psoriasis; preceding streptococcal pharyngitis
D) Nummular eczema; history of atopic dermatitis
E) Tinea corporis; central clearing with active border

Correct Answer: A) Pityriasis rosea; herald patch

Explanation

This presentation is classic for pityriasis rosea (PR), a common, self-limiting papulosquamous disorder.

Key Clinical Features of Pityriasis Rosea:

• Herald Patch: The initial lesion is a single, round or oval, erythematous, scaly plaque (2-10 cm), typically appearing on the trunk or proximal extremities. This is the characteristic antecedent finding.

• Secondary Eruption: Within 1-2 weeks, a generalized eruption develops, featuring numerous smaller, pink, oval papules or plaques with a collarette of scale.

• Distribution: The secondary rash follows Langer's lines (skin cleavage lines) on the trunk, often creating a "Christmas tree" pattern on the back.

• Symptoms: Mild to moderate pruritus is common.

• Course: The rash typically resolves spontaneously within 6-8 weeks.

Etiology:

• Strongly associated with human herpesvirus 7 (HHV-7) and possibly HHV-6 reactivation. The herald patch may represent the initial viral replication site, with the generalized eruption reflecting an immune response.

Why Not the Other Options?

• (B) Secondary syphilis: Presents with copper-colored, scaly papules on the trunk, palms, and soles. Condylomata lata (moist, flat papules in intertriginous areas) are characteristic, not a herald patch. A history of a chancre may be reported.

• (C) Guttate psoriasis: Presents with small, droplike, scaly papules on the trunk and extremities, often triggered by streptococcal pharyngitis. There is no herald patch.

• (D) Nummular eczema: Features coin-shaped, erythematous, pruritic, scaly plaques often on the extremities. It is associated with dry skin and atopy, not a herald patch.

• (E) Tinea corporis: Presents as annular plaques with central clearing and an advancing, scaly border. It is not preceded by a herald patch and is usually asymmetric.

Management:

• Reassurance about the self-limited nature.

• Symptomatic relief: Emollients, topical corticosteroids for pruritus, and oral antihistamines.

• Phototherapy (UVB) may hasten resolution in severe cases.

Prognosis:
Excellent; recurrence is rare.

Note: The diagnosis of pityriasis rosea is clinical. The presence of a herald patch followed by a generalized rash along skin cleavage lines is pathognomonic. Always consider syphilis in the differential, especially in high-risk populations.