A term neonate develops lethargy, hypotonia, seizures, and apnea on day 2. Blood glucose and ammonia are normal, and there is no ketoacidosis. CSF analysis shows glycine of 900 micromol/L with a CSF-to-plasma glycine ratio of 0.15. MRI shows corpus callosum hypoplasia. The defective enzyme complex is:
- A Propionyl-CoA carboxylase
- B Serine dehydratase
- C Glycine cleavage complex ✓
- D Threonine aldolase
Correct answer: C. Glycine cleavage complex
Explanation
Nonketotic hyperglycinemia results from deficiency of the mitochondrial glycine cleavage system (P, H, T, L proteins), causing glycine accumulation in plasma and CSF. C CSF-to-plasma glycine ratio above 0.08 is diagnostic. Glycine acts as an excitatory neurotransmitter in cortex, explaining seizures and apnea. Normal glucose and absence of ketosis exclude organic acidemias such as propionic acidemia, which cause ketotic hyperglycinemia instead.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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