Final answer:
The most likely diagnosis based on the symptoms and findings is ulcerative colitis, an inflammatory bowel disease that primarily affects the colon and rectum and causes symptoms such as lower abdominal pain and bloody diarrhea.
Step-by-step explanation:
Based on the symptoms provided (diarrhea and bloody stool in the rectum and distal colon) and the findings from the colonoscopy (crypt abscesses), the most likely diagnosis is ulcerative colitis. Ulcerative colitis is an inflammatory bowel disease that primarily affects the colon and rectum, causing inflammation and ulcers in these areas. Its main symptoms include lower abdominal pain and bloody diarrhea. Unlike Crohn's disease, which can affect any part of the gastrointestinal tract, ulcerative colitis is limited to the colon and rectum.