A 38-year-old woman from rural Maharashtra presents with a 6-month history of progressive weakness, numbness in her feet, and difficulty walking. On examination, she has loss of vibration sense and proprioception in the lower limbs, hyperreflexia, and a positive Romberg sign. Her diet consists mainly of polished rice and legumes with minimal vegetables or animal products. Hemoglobin is 9.2 g/dL. What is the most likely vitamin deficiency?

## Clinical Diagnosis: Vitamin B12 Deficiency ### Key Clinical Features **Key Point:** The combination of subacute combined degeneration (SCD) with loss of vibration/proprioception, hyperreflexia, and positive Romberg sign is pathognomonic for B12 deficiency affecting the dorsal columns and spinocerebellar tracts. **High-Yield:** B12 deficiency causes a characteristic pattern: - **Dorsal column involvement** → loss of vibration sense and proprioception (early) - **Spinocerebellar tract involvement** → ataxia, positive Romberg - **Corticospinal tract involvement** → hyperreflexia and spasticity (distinguishes from B1 deficiency) - **Peripheral neuropathy** → distal paresthesias ### Why B12 and Not Other Causes? | Feature | B12 Deficiency | B1 Deficiency | B6 Deficiency | Folate Deficiency | |---------|---|---|---|---| | **Dorsal column signs** | ✓ (prominent) | ✗ | ✗ | ✗ | | **Hyperreflexia** | ✓ | ✗ (hyporeflexia) | ✗ | ✗ | | **Vibration/proprioception loss** | ✓ (early) | ✗ | ✗ | ✗ | | **Anemia** | ✓ (megaloblastic) | ✗ | ✗ | ✓ (megaloblastic) | | **Dietary risk (vegetarian/vegan)** | ✓ | ✗ | ✗ | ✗ | | **Romberg sign** | ✓ | ✗ | ✗ | ✗ | ### Mechanism of Neurological Damage **Clinical Pearl:** B12 is essential for myelin synthesis via methylmalonyl-CoA mutase and methionine synthase pathways. Deficiency causes demyelination of the dorsal and lateral columns, leading to SCD. This is **irreversible if untreated for >6 months**. **Mnemonic: "DORSAL" for B12 neuropathy:** - **D**orsal columns (vibration/proprioception) - **O**uter spinocerebellar (ataxia) - **R**oot/peripheral (paresthesias) - **S**pinal cord (SCD pattern) - **A**nemia (megaloblastic) - **L**oss of reflexes initially, then hyperreflexia (if cord involved) ### Risk Factors in This Patient 1. **Vegetarian diet** (no animal products) → B12 only in animal sources 2. **Polished rice** → loss of B vitamins during processing 3. **Rural setting** → limited access to fortified foods or supplements 4. **Mild anemia** (Hb 9.2) → consistent with megaloblastic anemia ### Diagnostic Confirmation **High-Yield:** Next steps would include: - Serum B12 level (low) - Methylmalonic acid and homocysteine (elevated in B12 deficiency) - Peripheral blood smear (macrocytic, hypersegmented neutrophils) - Intrinsic factor antibodies (if pernicious anemia suspected) [cite:Park 26e Ch 8]