A 28-year-old Indian male presents to the emergency department with acute hemolytic anemia following ingestion of fava beans at a family wedding. Laboratory investigations reveal hemoglobin 7.2 g/dL, reticulocyte count 8.5%, and serum bilirubin 3.8 mg/dL (predominantly unconjugated). Peripheral blood smear shows bite cells and Heinz bodies. Urine is dark brown. The patient has a history of similar episodes triggered by infections and certain medications. Which enzyme deficiency in the pentose phosphate pathway is most likely responsible for this clinical presentation?

Explanation

## Clinical Diagnosis: G6PD Deficiency **Key Point:** G6PD deficiency is the most common enzyme deficiency worldwide, affecting ~400 million people, with highest prevalence in Mediterranean, African, and Asian populations. It causes acute hemolytic crises triggered by oxidative stress (fava beans, sulfonamides, antimalarials, infections). ## Pathophysiology of G6PD Deficiency G6PD catalyzes the first committed step of the pentose phosphate pathway (PPP): $$\text{Glucose-6-phosphate} + \text{NADP}^+ \xrightarrow{\text{G6PD}} \text{6-phosphogluconolactone} + \text{NADPH}$$ **High-Yield:** NADPH is the critical reducing agent for: 1. **Glutathione reduction** — maintains reduced glutathione (GSH) pool 2. **Antioxidant defense** — GSH peroxidase neutralizes H₂O₂ and free radicals 3. **RBC membrane integrity** — prevents lipid peroxidation Without adequate NADPH → ↓GSH → oxidative stress → RBC hemolysis. ## Clinical Features Matching This Case | Finding | Mechanism | | --- | --- | | **Bite cells** | Macrophages remove Heinz bodies from RBCs, leaving "bites" | | **Heinz bodies** | Denatured hemoglobin precipitates (seen with supravital stain) | | **Dark urine** | Free hemoglobin → myoglobin-like appearance; elevated indirect bilirubin | | **Reticulocytosis** | Compensatory erythropoiesis after hemolysis | | **Fava bean trigger** | Vicine and convicine → oxidative stress in G6PD-deficient cells | **Clinical Pearl:** Hemolytic episodes are **episodic**, not chronic — between crises, patients are asymptomatic with normal hemoglobin. This distinguishes G6PD from hereditary spherocytosis or thalassemia. ## Why G6PD Deficiency Fits This Vignette 1. **Ethnic background:** Indian male — G6PD deficiency is common in South Asia 2. **Acute hemolysis post-exposure:** Classic presentation 3. **Bite cells + Heinz bodies:** Pathognomonic for G6PD 4. **Recurrent episodes with triggers:** History of similar events with infections/medications 5. **Unconjugated hyperbilirubinemia + reticulocytosis:** Hemolytic pattern ## Pentose Phosphate Pathway Context ```mermaid flowchart TD A["Glucose-6-phosphate"]:::outcome --> B["G6PD<br/>(Rate-limiting step)"]:::action B -->|"NADP+ → NADPH"| C["6-Phosphogluconolactone"]:::outcome C --> D["Pentose-5-phosphate<br/>(Nucleotide synthesis)"]:::outcome B -->|"NADPH regeneration"| E["Glutathione reduction<br/>(GSH pool)"]:::action E --> F["Antioxidant defense<br/>(H₂O₂ neutralization)"]:::action F --> G["RBC membrane protection"]:::outcome H["G6PD Deficiency"]:::urgent --> I["↓ NADPH production"]:::urgent I --> J["↓ GSH availability"]:::urgent J --> K["Oxidative stress (fava beans, drugs)"]:::urgent K --> L["Hemoglobin denaturation<br/>(Heinz bodies)"]:::urgent L --> M["Hemolysis"]:::urgent ``` **Mnemonic:** **G6PD = Glutathione Deficiency Prevention** — the enzyme prevents glutathione depletion by supplying NADPH.