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MCAD hypoketotic hypoglycaemia — MRCPCH TAS MCQ

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HardNutrition and metabolismMCAD hypoketotic hypoglycaemiaMRCPCH TAS

An 18-month-old becomes drowsy after a prolonged fast during gastroenteritis. Glucose is 1.9 mmol/L with low ketones, raised medium-chain acylcarnitines and mild hyperammonaemia. What is the most likely biochemical defect?

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Correct answer: A — Medium-chain fatty acid beta-oxidation defect

MCAD deficiency impairs fatty acid beta-oxidation, causing hypoketotic hypoglycaemia during fasting. Phenylalanine hydroxylase deficiency causes raised phenylalanine rather than fasting hypoketotic hypoglycaemia. The pearl is that avoidance of prolonged fasting is central to management of fatty acid oxidation disorders.

Reference: Nelson Textbook of Pediatrics; UK newborn screening resources; RCPCH Theory examination syllabi