A 3-month-old infant was cranky and irritable, became quite lethargic between feedings, and began to develop a potbelly. A physical examination demonstrated an enlarged liver, while blood work taken between feedings demonstrated elevated lactate and uric acid levels, as well as hypoglycemia. This child most likely has a mutation in which one of the following enzymes?

**Core Concept**
The infant's symptoms suggest a disorder of gluconeogenesis, a metabolic pathway that generates glucose from non-carbohydrate sources such as lactate, glycerol, and certain amino acids. This pathway is crucial for maintaining blood glucose levels during fasting periods.

**Why the Correct Answer is Right**
The symptoms described, including lethargy, hypoglycemia, and an enlarged liver, are characteristic of a deficiency in glucose-6-phosphatase, an enzyme essential for the final step of gluconeogenesis and glycogenolysis. This enzyme catalyzes the conversion of glucose-6-phosphate to glucose, which is then released into the bloodstream. A mutation in the glucose-6-phosphatase gene leads to a failure to properly regulate blood glucose levels, resulting in hypoglycemia and other metabolic disturbances.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyruvate carboxylase is an enzyme involved in gluconeogenesis, but a deficiency in this enzyme would result in a different set of symptoms, including lactic acidemia and ketosis.

**Option B:** Fructose-1,6-bisphosphatase is another enzyme involved in gluconeogenesis, but a deficiency in this enzyme would also result in a different set of symptoms, including lactic acidemia and hypoglycemia, but with a different clinical presentation.

**Option C:** Phosphoenolpyruvate carboxykinase is an enzyme also involved in gluconeogenesis, but a deficiency in this enzyme would result in a different set of symptoms, including lactic acidemia and hypoglycemia, but with a different clinical presentation.

**Option D:** Glucose-6-phosphatase is the correct answer.

**Clinical Pearl / High-Yield Fact**
A deficiency in glucose-6-phosphatase is a classic example of a disorder of gluconeogenesis, and it is essential to recognize the clinical presentation of this condition, including hypoglycemia, lactic acidemia, and an enlarged liver, to make an accurate diagnosis.

**Correct Answer:** D. Glucose-6-phosphatase.

✓ Correct Answer: C. Glucose-6-phosphatase