What is the most likely diagnosis for a 25-year-old patient with a 6-month history of bloody diarrhea, lower abdominal cramps, and mild left lower quadrant abdominal tenderness, with a leukocyte (white blood cell) count of 5.0?

Most Likely Diagnosis: Inflammatory Bowel Disease (Ulcerative Colitis)

This 25-year-old patient with 6 months of bloody diarrhea, lower abdominal cramps, and left lower quadrant tenderness most likely has inflammatory bowel disease, specifically ulcerative colitis. The chronic duration, bloody diarrhea, and localization to the left colon are classic features that distinguish this from infectious or functional causes 1.

Key Clinical Features Supporting IBD

The constellation of symptoms points strongly toward organic inflammatory disease rather than functional disorders:

• Bloody diarrhea for 6 months is the hallmark presentation of ulcerative colitis, which typically manifests with bloody diarrhea, urgency, rectal bleeding, and abdominal pain confined to the colon 1.

• Left lower quadrant tenderness corresponds anatomically to left-sided colitis, the most common distribution pattern in ulcerative colitis 1.

• Normal leukocyte count (5.0) does not exclude IBD, as up to 15-20% of patients with active Crohn's disease have normal inflammatory markers, and this applies to ulcerative colitis as well 1, 2.

• Chronic duration (6 months) strongly favors IBD over infectious causes, which typically resolve within days to weeks 3.

Why This is NOT Irritable Bowel Syndrome

IBS diagnosis requires the absence of alarm features, and this patient has multiple red flags 1, 2:

• Bloody stools (major alarm feature)
• Chronic duration with persistent symptoms
• Abdominal tenderness on examination
• Age 25 with new-onset symptoms

IBS symptoms are relieved with defecation, associated with changes in stool frequency or form, and include bloating without systemic symptoms—none of which includes bloody diarrhea 1.

Why This is NOT Infectious Diarrhea

The 6-month duration effectively excludes acute infectious causes 3:

• Bacterial enteritis (Shigella, Salmonella, Campylobacter) typically resolves within 1-2 weeks even without treatment 3.
• While these infections share inflammatory features (fever, abdominal pain, bloody stools), the chronic relapsing nature over 6 months is inconsistent with self-limited infection 3.
• Persistent diarrhea beyond 14 days should prompt evaluation for non-infectious causes including IBD 3.

Essential Next Steps for Diagnosis Confirmation

Immediate laboratory workup should include 1, 2, 4:

1. Fecal calprotectin (93-95% sensitivity and 91-96% specificity for IBD):

   • Values >200-250 μg/g strongly suggest IBD and mandate colonoscopy 1, 2, 4
   • Values <50 μg/g would effectively exclude IBD 1, 2, 4

2. Complete blood count to assess for anemia (common in chronic bloody diarrhea) and inflammatory changes 1, 2.

3. C-reactive protein or ESR to detect systemic inflammation, though normal values do not exclude active disease 1, 2.

4. Stool studies:

   • Clostridioides difficile toxin (mandatory in all new diarrhea presentations) 2, 4
   • Stool culture to exclude bacterial pathogens 2, 4
   • Fecal occult blood (likely positive given bloody diarrhea) 2, 4

5. Celiac serology (IgA tissue transglutaminase with total IgA) to exclude celiac disease, which can mimic IBD 1.

Colonoscopy with biopsies is the definitive diagnostic test 1, 5:

• Required to confirm IBD diagnosis through visualization of mucosal inflammation and histological examination 1, 5.
• Biopsies should be obtained from both affected and normal-appearing areas 2.
• Allows assessment of disease extent and distribution (proctitis, left-sided colitis, or extensive colitis) 5.

Critical Diagnostic Pitfalls to Avoid

• Do not dismiss this as IBS based on normal white blood cell count alone—bloody diarrhea is never a feature of IBS and mandates full IBD workup 1.

• Do not assume infectious cause has been excluded without performing stool studies, particularly C. difficile testing which is mandatory regardless of antibiotic history 2, 4.

• Do not delay colonoscopy if fecal calprotectin is elevated >200 μg/g or if clinical suspicion remains high despite intermediate values 2, 4.

• Do not rely solely on inflammatory markers (WBC, CRP) to exclude IBD, as 15-20% of patients with active disease have normal values 1, 2, 4.