Mrs. Anderson displays an itchy raised rash that appears and disappears at various locations. What is the most likely cause of this rash?

The presence of an itchy raised rash that appears and disappears at various locations strongly suggests urticaria, commonly known as hives. Urticaria is characterized by the development of wheals—raised, itchy areas of skin that result from the release of histamine and other chemicals, often due to an allergic reaction or irritant. This rash can change in location and size rapidly, with the ability to resolve and then reappear, which aligns well with the description provided.

In contrast, other conditions like psoriasis tend to produce more persistent plaques that do not fluctuate in appearance and location as much. Allergic reactions may cause rashes but typically present with additional symptoms, and folliculitis is more localized around hair follicles, primarily resulting in red, inflamed bumps rather than a widespread itchy rash that moves around. Thus, the pattern of the rash described in Mrs. Anderson's case points towards urticaria being the most likely cause.