An 18‑year‑old thin female presents with chronic watery diarrhea, intermittent abdominal pain, pallor, macrocytic anemia due to low vitamin B12 (normal folate), elevated platelet count, normal white blood cell count, and no history of gastrointestinal bleeding; which diagnosis is most likely: celiac disease, Crohn’s disease, ulcerative colitis, or Whipple’s disease?

Crohn's Disease

The most likely diagnosis is Crohn's disease (B), given the combination of chronic diarrhea, abdominal pain, macrocytic anemia with isolated vitamin B12 deficiency (normal folate), thrombocytosis, and young age in a thin female patient. 1, 2

Key Diagnostic Features Supporting Crohn's Disease

Laboratory Pattern Analysis

• Isolated vitamin B12 deficiency with normal folate is highly characteristic of ileal involvement in Crohn's disease, as the terminal ileum is the specific absorption site for B12 3, 4, 5
• Vitamin B12 deficiency occurs in 15.6% of Crohn's disease patients compared to only 2.8% in ulcerative colitis 4
• The macrocytic anemia (MCV 100 fl) with low B12 (0.074 μmol/L, well below normal 0.12-0.66) directly indicates terminal ileal disease 3, 5

Thrombocytosis as an Inflammatory Marker

• The elevated platelet count (540 × 10⁹/L) reflects active chronic inflammation, which is common in Crohn's disease 1, 6
• Thrombocytosis serves as an acute phase reactant indicating ongoing disease activity 6

Clinical Presentation Alignment

• Chronic diarrhea and intermittent abdominal pain without GI bleeding fits the typical presentation of small bowel Crohn's disease 1, 2
• The patient's slim build and pallor suggest chronic malabsorption and nutritional deficiency, consistent with small bowel involvement 1, 7

Why Other Diagnoses Are Less Likely

Celiac Disease (A) - Excluded

• Celiac disease typically causes both folate AND B12 deficiency due to proximal small bowel involvement affecting the duodenum and jejunum where folate is absorbed 1
• The normal folate level (0.099 μmol/L, borderline but not deficient) argues strongly against celiac disease 1
• Celiac disease would be expected to show low folate first, as the duodenum/jejunum are primarily affected 1

Ulcerative Colitis (C) - Excluded

• Ulcerative colitis is a colonic disease that does not cause vitamin B12 deficiency, as B12 absorption occurs in the terminal ileum 4, 7
• B12 deficiency prevalence in UC is only 2.8% versus 15.6% in Crohn's disease 4
• The isolated B12 deficiency pattern is incompatible with UC 4, 7

Whipple's Disease (D) - Excluded

• Whipple's disease is extremely rare, particularly in young women (typically affects middle-aged men) 1
• Would require PAS-positive macrophages on biopsy and PCR for Tropheryma whipplei for diagnosis 1
• The clinical presentation lacks the characteristic arthralgia and systemic features typical of Whipple's disease 1

Critical Diagnostic Considerations

Risk Factors for B12 Deficiency in Crohn's Disease

• Ileal resection (even ≤20 cm increases risk 3-fold; >20 cm increases risk 6.7-fold) 5
• Active ileal inflammation increases risk 3.9-fold 5
• Disease location: Terminal/distal ileal involvement shows significantly lower B12 levels 3

Recommended Next Steps

• Complete ileocolonoscopy with systematic biopsies from at least five sites including terminal ileum and rectum, even from normal-appearing mucosa 2
• Cross-sectional imaging (MR enterography preferred over CT due to lack of radiation in young patient) to assess small bowel involvement, strictures, and disease extent 2, 6
• Stool studies to exclude infectious causes (C. difficile, common pathogens) before confirming IBD diagnosis 2, 6
• Additional laboratory tests: C-reactive protein, ESR, albumin, fecal calprotectin (sensitivity 93%, specificity 96% for IBD), and iron studies 2, 6

Common Pitfall to Avoid

• Do not assume celiac disease based solely on chronic diarrhea and anemia in a young female—the isolated B12 deficiency with normal folate is the key discriminating feature pointing to terminal ileal pathology characteristic of Crohn's disease 3, 4, 5