1. A 41-year-old man presents to his physician complaining of swollen ankles. He states that he has never had this problem before. Physical examination demonstrates marked lower extremity edema and periorbita l swelling. His pulse is strong, and regular. Urine dipstick is 4+ positive for protein, but negative for blood and glucose. 24-hour urine collection demonstrates proteinuria of 6 grams per day. Which of the following is the most likely cause of this patient's disorder?