What is the most likely diagnosis for a young woman with a mildly itchy rash characterized by salmon-colored plaques with fine scale?

In this scenario, the description of a mildly itchy rash with salmon-colored plaques and fine scaling is most characteristic of pityriasis rosea. This condition typically presents with a "herald patch," which is a larger solitary lesion appearing first, followed by smaller lesions that develop in a pattern often resembling a Christmas tree on the trunk.

Pityriasis rosea is known for its seasonal occurrence and usually resolves on its own within a few weeks to months. The salmon-colored plaques are a hallmark of this condition and differentiate it from other skin disorders.

While conditions like psoriasis can present with scaling and erythematous plaques, the specific characteristics, distribution, and the presence of the herald patch help confirm pityriasis rosea as the most likely diagnosis in this case.

Tinea versicolor typically doesn't present with salmon-colored plaques but rather lighter or darker spots on the skin and is caused by a fungal infection. Seborrheic dermatitis usually involves greasy, scaly patches often on the scalp and face rather than clearly defined plaques throughout the body. Thus, in the context provided, pityriasis rosea stands out as the most fitting diagnosis.