A 48-year-old patient presents with fever, abdominal pain, and bloody diarrhea. What is the most likely diagnosis and test to order?

The most likely diagnosis for the patient’s symptoms of fever, abdominal pain, and bloody diarrhea is ulcerative colitis. This inflammatory bowel disease typically presents with these clinical features, particularly bloody diarrhea, as it primarily affects the colon and can cause significant inflammation.

Ordering a sigmoidoscopy is the appropriate test in this case because it allows direct visualization of the rectum and sigmoid colon, where ulcerative colitis most commonly manifests. During the procedure, tissue samples can be obtained if necessary for histological examination, which assists in confirming the diagnosis of ulcerative colitis.

Other options, while related to gastrointestinal issues, do not align as closely with the specific symptoms presented. Cholecystitis usually presents with right upper quadrant pain and may not typically cause bloody diarrhea. Cirrhosis is more related to liver dysfunction and presents with different symptoms. Gastroenteritis might cause diarrhea but would less likely result in the pronounced bloody diarrhea described, and stool culture would be more appropriate for infectious causes rather than an inflammatory condition like ulcerative colitis.