Correct Answer: A. Hexose monophosphate (HMP) pathway
This question tests recognition of G6PD deficiency (glucose-6-phosphate dehydrogenase deficiency), the most common enzyme defect worldwide affecting ~400 million people, with high prevalence in Mediterranean, African, and Asian populations. Primaquine is an oxidative stressor that precipitates hemolysis in G6PD-deficient individuals. The HMP pathway (pentose phosphate pathway) is the primary source of NADPH, which reduces glutathione (GSH) to maintain its antioxidant capacity. Without adequate NADPH production, RBCs cannot regenerate reduced glutathione, leading to oxidative damage and hemolysis. G6PD catalyzes the first committed step of the HMP pathway (glucose-6-phosphate → 6-phosphogluconolactone), making it essential for antioxidant defense in RBCs. Primaquine, sulfonamides, aspirin, and fava beans are classic oxidative triggers in G6PD deficiency. The hemolytic episode typically occurs 24–72 hours after drug exposure and is self-limited as older RBCs (which have lower G6PD activity) are preferentially destroyed, leaving younger RBCs with higher enzyme levels. This is a high-yield concept in Indian medical education and critical for clinical practice in endemic regions.
Why the other options are wrong
B. Gluconeogenesis — Gluconeogenesis is involved in glucose synthesis from non-carbohydrate sources and plays no role in antioxidant defense or RBC protection. While it maintains blood glucose, it has no relationship to oxidative stress or hemolysis. This is a distractor that tests whether students confuse metabolic pathways with protective mechanisms. C. Luebering-Rapoport pathway — The Luebering-Rapoport pathway (Rapoport-Luebering shunt) diverts glycolytic intermediates to regulate hemoglobin-oxygen affinity via 2,3-BPG production. It does not generate NADPH or provide antioxidant defense. This is a classic NBE trap—students may recall this pathway as RBC-specific but confuse its function with antioxidant protection. D. Glycolysis — Glycolysis generates ATP for RBC energy but does not produce NADPH. While glycolysis is essential for RBC survival, it cannot compensate for G6PD deficiency because the HMP pathway is the exclusive source of NADPH in RBCs. Glycolytic defects cause hemolysis through ATP depletion, not oxidative stress.
High-Yield Facts
- G6PD deficiency is X-linked recessive; males are predominantly affected; females are usually carriers but can show symptoms if homozygous or with unfavorable X-inactivation.
- NADPH from the HMP pathway is the sole source of reducing power for glutathione reduction; without it, RBCs cannot neutralize oxidative stress.
- Primaquine, sulfonamides, aspirin, and fava beans are classic oxidative triggers; hemolysis is typically acute and self-limited (older RBCs destroyed first).
- Heinz bodies (denatured hemoglobin precipitates) are pathognomonic on supravital staining in acute G6PD hemolysis.
- HMP pathway is the only source of NADPH in mature RBCs (which lack mitochondria); this makes it irreplaceable for antioxidant defense.
- Neonatal jaundice and acute hemolytic episodes are the two main clinical presentations of G6PD deficiency in India and endemic regions.
Mnemonics
G6PD Triggers: SAPS Sulfonamides, Aspirin, Primaquine, Severe infection (and fava beans). These are the classic oxidative stressors that precipitate hemolysis in G6PD deficiency. HMP Pathway Function: NADPH for Antioxidants The pentose phosphate pathway's sole purpose in RBCs is to generate NADPH → reduces GSH → neutralizes ROS. No NADPH = no antioxidant defense = hemolysis.
NBE Trap
NBE pairs primaquine (an antimalarial) with hemolytic anemia to test whether students reflexively think of sickle cell disease or thalassemia (common hemolytic anemias in India) rather than recognizing the drug-induced oxidative stress pattern specific to G6PD deficiency. The Mediterranean origin is a deliberate clue to G6PD prevalence in that region.
Clinical Pearl
In Indian practice, G6PD deficiency is a leading cause of neonatal jaundice in endemic regions (especially among Scheduled Castes and Tribes in South India). A Mediterranean or African patient presenting with acute hemolysis after primaquine is a classic board scenario—the diagnosis hinges on recognizing that oxidative stress overwhelms the HMP pathway's antioxidant capacity, not a defect in glucose production or ATP synthesis.
_Reference: Robbins Ch. 12 (Red Blood Cell Disorders); KD Tripathi Ch. 12 (Carbohydrate Metabolism); Harrison Ch. 108 (Hemolytic Anemias)_