A 12-year-old girl presents with itchy lesions on her trunk and scalp. What is her most likely diagnosis?

In this scenario, the presentation of itchy lesions on the trunk and scalp in a 12-year-old girl most strongly aligns with psoriasis. Psoriasis is a chronic skin condition characterized by red, scaly plaques that can appear on various parts of the body, including the trunk and scalp. This condition can also be associated with intense itching, which fits with the presentation described in the question.

Typically, psoriasis involves well-defined lesions, often exhibiting silvery-white scales, and can be exacerbated by stress, infections, or other environmental factors.

Atopic dermatitis is generally more associated with flexural surfaces, such as the insides of the elbows and knees, rather than lesions predominantly on the trunk and scalp. Pityriasis rosea usually presents with a herald patch followed by a more generalized rash and is less itchy, while tinea corporis, a fungal infection, tends to have a ring-like appearance and often is more commonly seen in a generalized pattern with defined borders, particularly in warm, humid environments.

Therefore, taking into account the location of the lesions, the age of the patient, and the nature of the itching, psoriasis is the most likely diagnosis in this case.