What is the most likely diagnosis for a slim patient with pallor, presenting with chronic diarrhea, intermittent abdominal pain, low hemoglobin (Hb), microcytic anemia (low MCV), thrombocytosis (elevated platelet count), and deficiencies in vitamin B12 and folic acid, with no history of gastrointestinal (GI) bleeding?

Most Likely Diagnosis: Celiac Disease

The most likely diagnosis is celiac disease (Option A), given the combination of chronic diarrhea, malabsorption features (low B12 and folate), anemia with normal MCV, and thrombocytosis in a slim patient with pallor.

Clinical Reasoning

Key Diagnostic Features Supporting Celiac Disease

The patient presents with a classic constellation of findings that strongly suggest small bowel enteropathy, specifically celiac disease:

• Iron deficiency is a sensitive indicator of small bowel enteropathy, particularly celiac disease 1
• Diarrhea is present in 43-85% of patients with newly diagnosed celiac disease 1
• The prevalence of celiac disease in patients referred to secondary care with chronic diarrhea ranges from 3-10% 1
• Small bowel biopsies reveal celiac disease in 2-3% of patients presenting with iron deficiency anemia 1

Laboratory Pattern Analysis

The laboratory findings are particularly revealing:

• Anemia (Hb 100 g/L) with normal MCV (100 fl): While the MCV is at the upper limit of normal, this represents combined deficiencies rather than pure macrocytic anemia 2
• Combined B12 and folate deficiency: Both vitamins are absorbed in the proximal small intestine, which is the primary site affected in celiac disease 3, 4
• Thrombocytosis (540 x 10⁹/L): Reactive thrombocytosis commonly occurs with chronic inflammation and iron deficiency, frequently seen in celiac disease 3
• Pallor and slim build: Consistent with chronic malabsorption and malnutrition typical of untreated celiac disease 3, 4

Why Not the Other Options?

Crohn's Disease (Option B):

• While Crohn's disease can cause chronic diarrhea and malabsorption, it typically presents with younger patients having Dieulafoy's lesion and Crohn's disease as causes of GI bleeding 1, 5
• The absence of GI bleeding history makes Crohn's less likely, as inflammatory bowel disease often presents with occult or overt bleeding
• Crohn's disease more commonly causes isolated B12 deficiency (terminal ileum involvement) rather than combined B12 and folate deficiency 1

Ulcerative Colitis (Option C):

• Ulcerative colitis affects the colon, not the small intestine where B12 and folate are absorbed
• Would not explain the combined vitamin deficiencies
• Typically presents with bloody diarrhea rather than malabsorption 6

Whipple's Disease (Option D):

• Extremely rare condition
• Usually presents with additional systemic features (arthralgia, fever, lymphadenopathy) not mentioned here 1

Recommended Diagnostic Approach

Immediate Testing

Serological testing for celiac disease is mandatory in patients presenting with chronic diarrhea 1:

• Tissue transglutaminase antibody (TTG IgA) with total IgA level 1
• Endomysial antibody (EMA) for combined sensitivity and specificity over 90% 1
• If IgA deficient (occurs in 2.6% of celiac patients), obtain IgG EMA or IgG TTG 1

Endoscopic Evaluation

• Upper GI endoscopy with duodenal biopsies should be performed even if antibodies are negative, as antibody-negative celiac disease accounts for 6.4-7% of cases 1
• Small bowel biopsies should be taken during endoscopy as 2-3% of patients presenting with iron deficiency anemia have celiac disease 1

Additional Baseline Testing

A basic screen for evidence of malabsorption should include: full blood count, urea and electrolytes, liver function tests, vitamin B12, folate, calcium, ferritin, erythrocyte sedimentation rate, and C-reactive protein 1

Critical Pitfalls to Avoid

• Do not give folic acid supplementation exceeding 0.4 mg daily until pernicious anemia is ruled out, as folic acid may mask B12 deficiency while allowing neurologic damage to progress 7, 8
• Do not assume dietary deficiency alone explains the findings - full GI investigation is required even with positive dietary history 1
• Do not accept upper GI lesions like erosions or peptic ulcers as the sole cause without completing lower GI evaluation, as dual pathology occurs in 10-15% of patients 1
• Vitamin B12 deficiency allowed to progress for longer than 3 months may produce permanent degenerative lesions of the spinal cord 7