A 58-year-old Indian woman presents with progressive fatigue, dyspnea on exertion, and paresthesias in her feet for 3 months. She is a strict vegetarian who avoids dairy products. On examination, she has glossitis, mild jaundice, and diminished vibration sense in the lower limbs. Her hemoglobin is 7.2 g/dL, MCV is 112 fL, and reticulocyte count is 0.8%. Peripheral blood smear shows hypersegmented neutrophils and macro-ovalocytes. Serum vitamin B12 level is 180 pg/mL (normal: 200–900 pg/mL). What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Vitamin B12 Deficiency Megaloblastic Anemia ### Key Clinical Features **Key Point:** This patient presents with the classic triad of B12 deficiency: macrocytic anemia, glossitis, and peripheral neuropathy (paresthesias with diminished vibration sense). **High-Yield:** The combination of: - Macrocytic anemia (MCV 112 fL, Hb 7.2 g/dL) - Hypersegmented neutrophils (>5 lobes) on blood smear - Macro-ovalocytes - Low serum B12 (180 pg/mL, below normal range) - Neurological signs (paresthesias, diminished vibration sense — dorsal column involvement) - Glossitis (glossal atrophy and inflammation) ...is pathognomonic for B12 deficiency megaloblastic anemia. ### Pathophysiology **Key Point:** Vitamin B12 is essential for: 1. DNA synthesis (via methylmalonyl-CoA mutase and methionine synthase pathways) 2. Myelin formation in the nervous system Deficiency causes: - **Impaired DNA synthesis** → megaloblastic erythropoiesis (large, immature RBCs) - **Impaired myelin synthesis** → subacute combined degeneration (SCD) of the spinal cord ### Why B12 Deficiency, Not Folate? | Feature | B12 Deficiency | Folate Deficiency | |---------|---|---| | **Neurological signs** | Present (paresthesias, vibration loss, ataxia) | Absent | | **Glossitis** | Yes (atrophic) | Yes (beefy red) | | **Serum B12** | Low (<200 pg/mL) | Normal | | **Serum folate** | Normal or high | Low | | **Dietary risk** | Strict vegetarian/vegan | Alcoholism, poor diet | **Clinical Pearl:** Neurological manifestations are the **distinguishing feature** of B12 deficiency. Folate deficiency does NOT cause neuropathy or dorsal column signs. ### Etiology in This Patient Strict vegetarian diet avoiding dairy = **dietary B12 deficiency** (pernicious anemia from autoimmune gastritis is the most common cause in developed countries, but dietary deficiency is common in India among vegetarians). ### Laboratory Interpretation **Key Point:** - Serum B12 = 180 pg/mL (low; normal >200) - MCV = 112 fL (macrocytic; normal <100) - Reticulocyte count = 0.8% (inappropriately low for the degree of anemia — indicates ineffective erythropoiesis) - Hypersegmented neutrophils = hallmark of megaloblastic maturation **High-Yield:** The **low reticulocyte count despite severe anemia** is a key clue to megaloblastic anemia (not hemolysis, where reticulocytes would be elevated). ### Management Algorithm ```mermaid flowchart TD A[Macrocytic anemia + neurological signs]:::outcome --> B{Serum B12 level?}:::decision B -->|Low| C[B12 deficiency confirmed]:::outcome C --> D{Cause of B12 deficiency?}:::decision D -->|Dietary/malabsorption| E[Parenteral B12 therapy]:::action D -->|Pernicious anemia| F[Lifelong IM B12 monthly]:::action E --> G[Neurological recovery if caught early]:::outcome F --> G ``` **Warning:** Neurological damage becomes **irreversible** if B12 deficiency is not treated promptly. Early diagnosis and treatment are critical. **Clinical Pearl:** Folate supplementation alone in B12 deficiency can mask the hematological findings while neurological damage continues — this is why B12 must be checked before empirical folate therapy.