Correct Answer: C. Thiamine-transketolase
Polished rice has the bran and germ removed—the primary sources of B vitamins in rice. This results in thiamine (vitamin B₁) deficiency, leading to beriberi in endemic areas like parts of India where polished rice is the staple carbohydrate. Thiamine functions as a coenzyme in the form of thiamine pyrophosphate (TPP), which is essential for three key enzymatic reactions: (1) transketolase in the pentose phosphate pathway, (2) pyruvate dehydrogenase in carbohydrate metabolism, and (3) α-ketoglutarate dehydrogenase in the TCA cycle. When thiamine is deficient, transketolase activity is impaired first and most severely because it is the most sensitive indicator of thiamine status—this is why transketolase activity (measured by the erythrocyte transketolase test) is used clinically to diagnose thiamine deficiency. Polished rice consumption does not cause riboflavin deficiency because riboflavin is also present in other dietary sources (milk, eggs, leafy greens) commonly consumed alongside rice in Indian diets. The combination of thiamine deficiency with transketolase enzymatic defect is pathognomonic for polished rice-dependent populations.
Why the other options are wrong
A. Riboflavin-glutathione reductase — This is incorrect because polished rice consumption causes thiamine deficiency, not riboflavin deficiency. While riboflavin (B₂) is removed during polishing, it is present in sufficient quantities in other common Indian foods (milk, yogurt, leafy greens). Glutathione reductase is the enzyme dependent on riboflavin (FAD), but this pairing does not match the primary deficiency caused by polished rice alone. The trap is pairing a real enzyme with the wrong vitamin. B. Thiamine-transaminase — This is incorrect because transaminase enzymes depend on pyridoxal phosphate (vitamin B₆), not thiamine. Although thiamine is correctly identified as the deficient vitamin in polished rice consumption, transaminase is not a thiamine-dependent enzyme. This option exploits the student's knowledge that thiamine is deficient but tests whether they know which specific enzymes require TPP as a cofactor. Transaminase deficiency would occur with B₆ deficiency, not B₁. D. Riboflavin-transketolase — This is incorrect because transketolase requires thiamine pyrophosphate (TPP), not riboflavin (FAD) as its cofactor. While transketolase is the correct enzyme, riboflavin is the wrong vitamin. This is a classic NBE trap: pairing the correct enzyme with an incorrect vitamin to test whether students understand the specific coenzyme requirements of each enzyme. Polished rice causes thiamine deficiency, not riboflavin deficiency.
High-Yield Facts
- Thiamine (B₁) deficiency is the primary consequence of consuming only polished rice because the bran and germ—the richest sources—are removed during milling.
- Transketolase is the most sensitive enzyme to thiamine deficiency and is used clinically (erythrocyte transketolase test) to diagnose B₁ deficiency.
- Thiamine pyrophosphate (TPP) is the active coenzyme form required for transketolase, pyruvate dehydrogenase, and α-ketoglutarate dehydrogenase.
- Beriberi (wet or dry form) is the clinical manifestation of severe thiamine deficiency, historically endemic in polished rice-consuming populations in Asia.
- Riboflavin (B₂) is not typically deficient in polished rice diets because it is available from milk, eggs, and leafy vegetables commonly consumed in Indian households.
Mnemonics
TPP Enzymes (Thiamine-dependent) Transketolase, Pyruvate dehydrogenase, Phosphoketolase, α-Ketoglutarate dehydrogenase, Branched-chain ketoacid dehydrogenase. Remember: Transketolase is the first to fail in thiamine deficiency. Polished Rice → Thiamine Loss Bran & Germ = B vitamins. Polishing removes these layers → thiamine deficiency → beriberi. Riboflavin is spared because it comes from other sources (dairy, greens).
NBE Trap
NBE pairs riboflavin with transketolase (option D) to exploit students who know transketolase is important in thiamine deficiency but confuse which B vitamin is the cofactor. The trap tests coenzyme-enzyme specificity, not just vitamin knowledge.
Clinical Pearl
In Indian populations with high polished rice consumption (particularly in South India and parts of Northeast India), thiamine deficiency manifests as Wernicke-Korsakoff syndrome in acute presentations or chronic dry beriberi with peripheral neuropathy. The erythrocyte transketolase test (measuring TPP effect) is the gold standard for diagnosing thiamine deficiency in such populations and guides supplementation strategies in public health programs.
_Reference: Harper Biochemistry Ch. 44 (Vitamins); KD Tripathi Pharmacology Ch. 48 (Vitamins)_