Quiz Corner

A 12-year-old Chihuahua is presented for hair loss. Examination reveals a multifocal circular alopecia with evidence of papules and pustules, but no evidence of scratching. The skin of the abdomen is extremely thin with abundant comedones and prominent blood vessels. Upon further questioning of the owner it is revealed that the patient has been urinating in the house and drinking increased amounts of water. Which of the following is the most appropriate test to confirm the most likely diagnosis