A 76-year-old man with a slowly enlarging leg plaque likely has which skin condition?

The condition described—a slowly enlarging leg plaque in a 76-year-old man—aligns well with Bowen's disease. Bowen's disease is a form of squamous cell carcinoma in situ that presents as a persistent, often enlarging, erythematous plaque. It frequently occurs on sun-exposed areas, such as the legs, particularly in older adults. The key characteristic is the gradual enlargement of the lesion, which, in this case, indicates a potential malignancy, making it vital to identify and treat early.

In contrast, psoriasis vulgaris typically manifests as well-defined, erythematous plaques covered with silvery scales and can have an acute rather than slowly enlarging course. Pityriasis rosea is usually self-limiting and presents with a herald patch followed by a generalized rash; it does not typically present as a slowly enlarging plaque. Discoid eczema, while presenting as plaques, is often associated with itching and is not predominantly characterized by slow enlargement in the same way as Bowen's disease.

Thus, given the patient's age and the description of the leg plaque, Bowen's disease is the most fitting diagnosis, emphasizing the importance of recognizing potential skin malignancies in elderly patients.