What is the most likely diagnosis for a patient with a history of bloody diarrhea, painful defecation, and mucus in stool, who previously used oral and rectal medications but stopped, and now presents with similar symptoms and abdominal tenderness?

Ulcerative Colitis

The most likely diagnosis is ulcerative colitis (UC), given the classic triad of bloody diarrhea, painful defecation with mucus, left lower quadrant tenderness, and a history of previous response to oral and rectal medications that were discontinued. 1, 2, 3

Clinical Presentation Supporting UC

The patient's symptom constellation is pathognomonic for ulcerative colitis:

• Bloody diarrhea with mucus is the hallmark presentation of UC, representing continuous mucosal inflammation starting from the rectum 3, 4, 5
• Painful defecation (tenesmus) with 3-4 bowel movements daily indicates active rectal inflammation 3, 4
• Left lower quadrant tenderness suggests distal colitis or proctosigmoiditis, the most common initial presentation of UC 1, 2
• Previous response to oral and rectal medications strongly suggests UC, as this treatment pattern (5-aminosalicylates) is first-line therapy for distal UC 1

Disease Relapse Pattern

This presentation represents a classic UC relapse after medication discontinuation:

• Approximately 50% of UC patients relapse annually, particularly after stopping maintenance therapy 6
• Discontinuation of oral and rectal 5-ASA therapy after symptom improvement is a common cause of disease flare 1
• The European Crohn's and Colitis Organisation emphasizes that patients with endoscopically documented distal colitis who previously responded to combined oral and rectal therapy have a predictable relapse pattern when medications are stopped 1

Differential Diagnosis Considerations

While UC is most likely, other diagnoses must be excluded:

• Infectious colitis (including C. difficile) must be ruled out with stool cultures, though the chronic relapsing pattern and previous medication response make this less likely 1, 6
• Crohn's disease is possible but less likely given the absence of perianal disease, skip lesions, or upper GI symptoms 2, 7
• Diverticulitis would typically present with more severe left lower quadrant pain, fever, and is uncommon in younger patients 2
• Ischemic colitis is unlikely given the patient's presumed younger age and chronic relapsing course 1

Diagnostic Confirmation Required

Flexible sigmoidoscopy with biopsies is the essential next step to confirm the diagnosis and assess disease severity:

• Sigmoidoscopy is sufficient for distal colitis and avoids the risks of full colonoscopy in active disease 1
• Expected endoscopic findings include continuous inflammation starting at the anal verge with clear demarcation, mucosal friability, spontaneous bleeding, and ulcerations 1
• Biopsies must be obtained to confirm chronic inflammation and exclude other causes 1
• Stool cultures for infectious pathogens (including C. difficile) should be obtained before endoscopy 1, 6

Critical Management Pitfalls to Avoid

• Do not perform full colonoscopy in acute colitis due to perforation risk; flexible sigmoidoscopy is adequate 1
• Do not use phosphate enemas or oral sodium phosphate preparations for bowel preparation in acute colitis 1
• Always exclude C. difficile infection before attributing symptoms to UC flare, as this can mimic or coexist with IBD 1, 8
• Do not delay reinitiation of therapy once infectious causes are excluded, as mucosal healing predicts better long-term outcomes 1