A rash appears suddenly on a young patient, characterized by multiple erythematous papules in a Christmas tree pattern. What is the likely diagnosis?

The presentation of multiple erythematous papules in a Christmas tree pattern is characteristic of pityriasis rosea. This skin condition typically begins with a single, larger lesion known as a "herald patch," followed by the development of smaller patches that appear in a pattern resembling the branches of a Christmas tree. Pityriasis rosea usually affects younger individuals and is often self-limiting, lasting several weeks to months.

The distinction of this rash pattern is a key diagnostic feature. Other conditions, such as eczema, psoriasis, and seborrheic dermatitis, have different appearances and symptoms. Eczema tends to cause more widespread and itchy lesions that may be scaly or weepy, while psoriasis is known for well-defined red plaques covered with silvery scales, typically located on extensor surfaces. Seborrheic dermatitis presents with greasy, yellowish scales and tends to affect areas rich in sebaceous glands, such as the scalp and face, rather than presenting in a Christmas tree-like distribution.

In summary, the sudden appearance of erythmatous papules arranged in a Christmas tree pattern strongly suggests pityriasis rosea, making it the most likely diagnosis in this scenario.