A 25-year-old HIV positive MSM was referred to you for a 1-month history of tenesmus, blood per rectum, pain, and fevers. On physical examination, small ulcerations of the perianal skin are noted. Flexible sigmoidoscopy reveals moderate proctitis but the descending colon mucosa is normal. Rectal biopsies reveal crypt abscesses and granulomas. He denies a family history of inflammatory bowel disease and of recent travel. What is the best next step in management?