A 42-year-old Indian man with a history of chronic diarrhea and steatorrhea for 2 years presents with progressive weakness and cognitive slowing. On examination, he has glossitis, mild ataxia, and loss of proprioception in the lower limbs. His hemoglobin is 8.5 g/dL, MCV is 108 fL, and WBC is 2800/μL with hypersegmented neutrophils. Serum folate is 2.1 ng/mL (normal: 5.4–16 ng/mL) and serum B12 is 95 pg/mL (normal: 200–900 pg/mL). Methylmalonic acid (MMA) is elevated at 3.8 μmol/L (normal: <0.4 μmol/L). Which of the following is the most likely underlying etiology?

Explanation

## Clinical Diagnosis: Tropical Sprue with Dual B12 and Folate Deficiency ### Key Clinical Features **High-Yield:** This patient has: - **Chronic diarrhea and steatorrhea** (2-year history) — hallmark of small bowel mucosal disease - **Dual deficiencies:** Low B12 (95 pg/mL) AND low folate (2.1 ng/mL) - **Megaloblastic anemia:** MCV 108, hypersegmented neutrophils - **Neurological involvement:** Ataxia, loss of proprioception (B12-related dorsal column involvement) - **Cognitive slowing:** Suggests B12 deficiency affecting CNS - **Elevated MMA:** Confirms B12 deficiency (MMA is a sensitive marker of B12 status) **Key Point:** The combination of **chronic diarrhea + BOTH low B12 and low folate + megaloblastic anemia** is the clinical signature of tropical sprue. ### Tropical Sprue: Pathophysiology **Mnemonic: SPRUE = Small Bowel Pathology Reducing Uptake Everywhere** Tropical sprue is a chronic small bowel mucosal disease (endemic in tropical regions: India, Caribbean, Southeast Asia) characterized by: 1. **Mucosal inflammation and atrophy** → reduced absorptive surface area 2. **Impaired absorption of BOTH folate and B12** (unlike most other causes, which affect only one) 3. **Malabsorption of fat, carbohydrates, and protein** → steatorrhea, diarrhea 4. **Possible infectious etiology** (Klebsiella, Enterobacteria, or other organisms) ### Why Tropical Sprue, Not Other Causes? | Feature | Tropical Sprue | Celiac | Crohn's | Pernicious Anemia | |---------|---|---|---|---| | **Diarrhea/steatorrhea** | Yes (prominent) | Yes | Yes (if small bowel) | No | | **B12 deficiency** | Yes | Rare | Yes (if terminal ileum) | Yes | | **Folate deficiency** | Yes (always) | Yes (common) | Possible | No | | **Both B12 + folate low** | **Yes** | Rare together | Possible | No (B12 only) | | **MMA elevated** | Yes | No | Possible | Yes | | **Geographic clue** | Tropical endemic | Any | Any | Any | | **Neurological signs** | Yes (B12 effect) | No | No | Yes | **Clinical Pearl:** Tropical sprue is the **only common cause of simultaneous B12 and folate deficiency** due to diffuse small bowel mucosal involvement. ### Why Not the Other Options? **Celiac Disease:** - Causes folate deficiency (common) - B12 deficiency is **rare** in celiac (terminal ileum usually spared) - Elevated MMA would not be expected - Diarrhea is present, but dual deficiency is atypical **Crohn's Disease:** - If terminal ileum involved → B12 malabsorption (yes) - Folate deficiency is possible but not as consistently as in tropical sprue - Dual deficiency is less common than in tropical sprue - Geographic clue (endemic in tropics) points away from Crohn's **Pernicious Anemia:** - Autoimmune gastritis → **B12 deficiency only** - Folate is normal or elevated (no mucosal disease) - No diarrhea or steatorrhea - No malabsorption of other nutrients ### Laboratory Interpretation **Key Point:** - **Serum B12 = 95 pg/mL** (low; normal >200) - **Serum folate = 2.1 ng/mL** (low; normal >5.4) - **Methylmalonic acid = 3.8 μmol/L** (elevated; normal <0.4) → confirms B12 deficiency - **MCV = 108 fL** (macrocytic) - **Hypersegmented neutrophils** → megaloblastic maturation - **WBC = 2800/μL** (mild leukopenia, common in megaloblastic anemia) **High-Yield:** The **elevated MMA is a sensitive and specific marker of B12 deficiency** — it rises when B12-dependent methylmalonyl-CoA mutase cannot function. ### Diagnostic Approach ```mermaid flowchart TD A[Chronic diarrhea + macrocytic anemia]:::outcome --> B{Serum B12 and folate?}:::decision B -->|Both low| C[Dual deficiency]:::outcome C --> D{MMA elevated?}:::decision D -->|Yes| E[B12 deficiency confirmed]:::outcome E --> F{Small bowel mucosal disease?}:::decision F -->|Yes - tropical endemic area| G[Tropical sprue]:::action F -->|Yes - non-tropical| H[Celiac or Crohn's]:::action F -->|No| I[Pernicious anemia unlikely]:::outcome ``` ### Management **Key Point:** Tropical sprue responds to: 1. **Antibiotics** (tetracycline or other broad-spectrum agents) — targets suspected bacterial overgrowth 2. **Folate supplementation** (5 mg daily) 3. **B12 supplementation** (IM or oral, depending on severity) 4. **Dietary support** and correction of other micronutrient deficiencies **Clinical Pearl:** Unlike pernicious anemia (which requires lifelong B12), tropical sprue may show **partial or complete remission** with antibiotics and nutritional support, suggesting an infectious or reversible mucosal etiology. **Warning:** If only B12 is supplemented without folate (or vice versa), the patient may not improve fully. Both must be corrected.