Pityriasis rosea is classically preceded by a herald patch and followed by patches in a Christmas tree pattern along skin lines. Which condition is this?

Pityriasis rosea is classically preceded by a herald patch and followed by patches in a Christmas tree pattern along skin lines. Which condition is this?

• A. Psoriasis
• B. Lichen planus
• C. Pityriasis rosea
• D. Seborrheic dermatitis

Answer: (C)

The key idea is recognizing pityriasis rosea by its distinctive sequence and pattern: a herald patch appears first, then a widespread eruption on the trunk that follows skin cleavage lines in a Christmas-tree arrangement. The herald patch is a single, larger oval plaque with a fine outer scale. Days to weeks later, multiple oval patches with collarette of scale appear on the trunk and spread downward in a diagonal fashion along the natural skin folds, creating a Christmas-tree pattern. This combination is characteristic of pityriasis rosea and helps separate it from other common dermatoses. Psoriasis tends to present with well-demarcated thick plaques with silvery scales on the elbows, knees, or scalp, often with nail changes, not a preceding herald patch or trunk patch pattern. Lichen planus shows pruritic purple polygonal papules with Wickham striae, typically on wrists, ankles, or mucous membranes, not a herald patch followed by trunk-line patches. Seborrheic dermatitis features greasy scales in areas like the scalp, face, or upper trunk, but lacks the herald patch and the Christmas-tree trunk distribution.

The key idea is recognizing pityriasis rosea by its distinctive sequence and pattern: a herald patch appears first, then a widespread eruption on the trunk that follows skin cleavage lines in a Christmas-tree arrangement. The herald patch is a single, larger oval plaque with a fine outer scale. Days to weeks later, multiple oval patches with collarette of scale appear on the trunk and spread downward in a diagonal fashion along the natural skin folds, creating a Christmas-tree pattern.

This combination is characteristic of pityriasis rosea and helps separate it from other common dermatoses. Psoriasis tends to present with well-demarcated thick plaques with silvery scales on the elbows, knees, or scalp, often with nail changes, not a preceding herald patch or trunk patch pattern. Lichen planus shows pruritic purple polygonal papules with Wickham striae, typically on wrists, ankles, or mucous membranes, not a herald patch followed by trunk-line patches. Seborrheic dermatitis features greasy scales in areas like the scalp, face, or upper trunk, but lacks the herald patch and the Christmas-tree trunk distribution.