After undergoing retropubic prostatectomy, a client returns to his room. the client is on nothing-by-mouth status and has an i. v. infusing in his right forearm at a rate of 100 ml/hour. the client also has an indwelling urinary catheter that's draining light pink urine. while assessing the client, the nurse notes that his urine output is red and has dropped to 15 ml and 10 ml for the last 2 consecutive hours. how can the nurse best explain this drop in urine output?