A 35-year-old man presents with recurrent episodes of hypoketotic hypoglycemia during fasting. Genetic testing reveals a loss-of-function mutation in CPT-1 (the enzyme marked **A** in the carnitine shuttle diagram). Which of the following best explains why this patient cannot mount an adequate ketogenic response during prolonged fasting?

Explanation

## Why option 1 is correct CPT-1 (carnitine palmitoyltransferase I) is the rate-limiting enzyme of fatty acid β-oxidation, catalyzing the first committed step: conversion of fatty acyl-CoA to acylcarnitine on the outer mitochondrial membrane (Harper 32e, Ch 22). Without functional CPT-1, long-chain fatty acids cannot enter the mitochondrial matrix for oxidation, even though they may be mobilized from adipose tissue. This blocks the substrate supply to the β-oxidation spiral and prevents the acetyl-CoA and NADH/FADH₂ needed to fuel ketogenesis. The patient presents with hypoketotic hypoglycemia because fasting-state ketone production is abolished, leaving only gluconeogenesis (which is limited by substrate availability) to maintain blood glucose. ## Why each distractor is wrong - **Option 2**: While carnitine-acylcarnitine translocase (B) does require acylcarnitine substrate, the primary defect is upstream at CPT-1. Translocase dysfunction would present similarly, but the question specifically anchors on CPT-1 deficiency. This is a mechanistically true statement but does not explain the CPT-1-specific pathophysiology. - **Option 3**: The β-oxidation spiral itself is intact in CPT-1 deficiency; the problem is substrate delivery, not enzyme activity. Fatty acyl-CoA does accumulate in the cytoplasm (not in the matrix where β-oxidation occurs), but this is a consequence, not the primary explanation for ketogenic failure. - **Option 4**: Malonyl-CoA is a physiologic inhibitor of CPT-1 in the fed state, but in fasting (when this patient is symptomatic), malonyl-CoA levels fall and CPT-1 is normally disinhibited. A genetic CPT-1 deficiency is not rescued by low malonyl-CoA; the enzyme is simply absent or non-functional. **High-Yield:** CPT-1 deficiency = impaired fatty acid entry into mitochondria → hypoketotic hypoglycemia (ketones low despite fasting). Malonyl-CoA inhibits CPT-1 in the fed state to prevent futile cycling of synthesis and oxidation. [cite: Harper 32e Ch 22 — Fatty Acid Oxidation and the Carnitine Shuttle; CPT-1 as rate-limiting enzyme and allosteric regulation by malonyl-CoA]