His head, ears, nose, throat, and neck examinations are normal. His cardiac, lung, and abdominal examinations are also normal. On visualization of the anus there is no inflammation, masses, or fissures. Digital rectal examination elicits an irregular, asymmetric, hard nodule on the otherwise normal posterior surface of the prostate. Examination of the scrotum and penis are normal. Laboratory results are pending. What disorder of the anus, rectum, or prostate is mostly likely in this case?