Final answer:
The patient's presentation of severe abdominal pain, distention, fever, tachycardia, and leukocytosis, in the setting of ulcerative colitis and recent antidiarrheal use, most likely indicates toxic megacolon.
Step-by-step explanation:
The most likely diagnosis for this patient with a history of ulcerative colitis who now presents with severe abdominal pain, a distended abdomen with markedly decreased bowel sounds, fever, tachycardia, and leukocytosis is E. Toxic megacolon. This condition is characterized by a severe dilatation of the colon seen in the context of inflammatory bowel diseases, like ulcerative colitis, and can be precipitated by the use of antidiarrheal medications such as loperamide. The absence of rebound tenderness or rigidity may exclude an acute perforation, and the remainder of the labs being normal makes acute pancreatitis and ischemic colitis less likely. In addition, since there's no mention of bloody diarrhea that would be characteristic of a Clostridioides difficile infection, option A is also less likely. Thus, with the clinical presentation and lab findings provided, toxic megacolon is the leading concern.