What is the most likely diagnosis for a 25-year-old man with a 9-day history of mucopurulent anal discharge, blood in stool, and pain while opening his bowels?

The clinical presentation of a 25-year-old man with a 9-day history of mucopurulent anal discharge, blood in stool, and pain during bowel movements is highly suggestive of a sexually transmitted infection, particularly gonorrhea. This condition can cause proctitis, which leads to mucopurulent discharge from the anal region, as well as rectal bleeding and pain during defecation.

Gonorrhea, caused by Neisseria gonorrhoeae, is known to infect the anal and rectal area in men, especially those who engage in receptive anal intercourse. The symptoms described, including the mucopurulent discharge and pain, align well with infections that affect the lower gastrointestinal tract, leading to inflammation and irritation.

The other diagnoses presented differ significantly in their typical presentations and causes. Crohn's disease and ulcerative colitis are inflammatory bowel diseases that might cause similar gastrointestinal symptoms but would not typically present with isolated mucopurulent anal discharge. Hemorrhoids can cause pain and blood in stool, but they usually do not present with significant discharge, especially not mucopurulent in nature. Thus, the specific combination of symptoms here strongly favors gonorrhea as the most likely diagnosis.