An Arabian horse with a dark proliferation around the anus most likely has which tumor?

• A. Melanoma
• B. Sarcoid
• C. Squamous cell carcinoma
• D. Lymphoma

Answer: (A)

Pigmented skin tumors in horses, especially around the perineal area, are most commonly melanomas, and this is particularly true in gray-patterned breeds such as Arabians. The dark proliferation near the anus fits the typical appearance of a melanocytic tumor, where melanin gives the lesion its deep color and nodular, sometimes ulcerated, growth around the mucocutaneous junction.

Sarcoids can occur in horses but are usually fibrous, nonpigmented or only variably pigmented nodules and are not characteristically located in this regional site. Squamous cell carcinoma tends to affect sun-exposed, pale areas such as the genitalia or eyelids and often presents as a crusted, ulcerated lesion rather than a deeply pigmented growth. Lymphoma would more likely present with multiple enlarging lymph nodes or systemic signs rather than a solitary perineal pigmented mass.

So the presentation most consistent with the described lesion is melanoma.