Final answer:
Abdominal distention is a symptom often associated with a serious complication of ulcerative colitis, such as toxic megacolon or perforation, and requires immediate medical attention.
Step-by-step explanation:
The symptom often associated with a serious complication of ulcerative colitis is C. Distention of the abdomen. Ulcerative colitis can lead to several serious complications, including severe bleeding, perforated colon, and increased risk of colon cancer. Abdominal distention could indicate complications like toxic megacolon or perforation, which requires immediate medical attention. Loose, blood-tinged stools and intense abdominal discomfort are common symptoms of ulcerative colitis but may not necessarily signify a severe complication. Decreased bowel sounds can occur as well but are less indicative of severe complications like toxic megacolon compared to abdominal distention.