An 85-year-old man presents to your office with complaints of intermittent rectal bleeding. His history is notable for prostate cancer treated with external beam radiation. Sigmoidoscopy reveals mucosal pallor, superficial ulceration, and telangiectasias in the distal rectum, and a biopsy specimen is obtained from the anterior rectal wall. Two weeks later, the patient develops pelvic pain, fever, and a watery rectal discharge that occurs while voiding. This presentation is most concerning for: