What is the most likely diagnosis for a rash characterized by salmon-colored plaques that follow the cleavage lines of the trunk?

The diagnosis of pityriasis rosea is supported by the presentation of salmon-colored plaques that typically follow the cleavage lines of the trunk, often referred to as the "Langer's lines." This condition commonly begins with a "herald patch," a single larger lesion that can appear before the rash becomes more widespread. The subsequent plaques may take on an oval shape and generally align with the skin's natural lines, which is characteristic of pityriasis rosea.

In contrast, tinea versicolor presents as hypopigmented or hyperpigmented macules and patches rather than salmon-colored plaques. It is caused by an overgrowth of Malassezia yeast and does not typically follow cleavage lines. Seborrheic dermatitis usually manifests as greasy, yellowish scales on erythematous skin and is often found in areas rich in sebaceous glands, such as the scalp and face, making it distinct from the presentation in this question. Psoriasis is characterized by well-defined, erythematous plaques covered with silvery scales, most commonly found on extensor surfaces rather than following the cleavage lines on the trunk.

Thus, considering the specific characteristics of the rash, the most likely diagnosis is pityriasis rosea, due to its distinct appearance and