**Question:** An infant at 7 months of age presented with history of vomiting and failure to thrive. Patient improved with administration of intravenous glucose and came out of coma within 24 hours. After one month he returned with similar complaints. On evaluation he is found to have raised blood ammonia and No ketones. Also, he has high urinary glutamine, alanine and uracil. Which is the likely enzyme defect in this patient?

A. Gluconeogenesis pathway defect
B. Glutamine synthetase defect
C. Glutamate dehydrogenase deficiency
D. Urea cycle defect

**Correct Answer:** D. Urea cycle defect

**Core Concept:** The urea cycle is a series of enzymatic reactions that convert ammonia produced from the breakdown of proteins to urea, which is then excreted in urine. The urea cycle is essential for maintaining ammonia homeostasis in the body. Ammonia is a toxic compound, and its accumulation can lead to severe neurological complications.

**Why the Correct Answer is Right:** In this case, the patient's clinical presentation, including raised blood ammonia, absence of ketones, and elevated urinary glutamine, alanine, and uracil, suggests a defect in the urea cycle. The high urinary excretion of these amino acids is characteristic of an impaired urea cycle, which is responsible for converting ammonia to urea. Incomplete urea cycle results in excess ammonia accumulation and neurological complications. Administration of intravenous glucose improved the child's condition, which is consistent with the urea cycle defect as glucose is used to fuel the cycle when ammonia levels are elevated.

**Why Each Wrong Option is Invalid:**

A. Gluconeogenesis pathway defect: This would result in hypoglycemia, not hyperammonemia, as gluconeogenesis produces glucose to maintain blood glucose levels, not urea.

B. Glutamine synthetase defect: This enzyme is involved in the synthesis of glutamine from ammonia, not urea production. It would lead to reduced glutamine levels, not elevated urinary glutamine as observed in this case.

C. Glutamate dehydrogenase deficiency: This enzyme participates in the conversion of glutamate to α-ketoglutarate, not urea production. A deficiency would result in reduced α-ketoglutarate levels, not elevated urinary glutamine, alanine, and uracil as seen in this case.

**Clinical Pearl:** Urea cycle disorders are a group of inborn errors of amino acid and nitrogen waste disposal, which can present with severe neurological symptoms like seizures, coma, and developmental regression. Early diagnosis and treatment can prevent long-term neurological impairment and improve outcomes.