In a 3-day-old infant with lethargy and poor feeding, what laboratory result is expected if serum ammonia is elevated?

In a 3-day-old infant presenting with symptoms such as lethargy and poor feeding, elevated serum ammonia can be indicative of a urea cycle disorder. In these conditions, the body is unable to effectively remove ammonia, leading to its accumulation in the bloodstream.

When serum ammonia levels rise, it can result in elevated levels of certain metabolites. In particular, elevated urine orotic acid is often associated with deficiencies in the urea cycle, such as ornithine transcarbamylase deficiency. This occurs because the urea cycle cannot effectively produce urea, leading to the diversion of excess carbamoyl phosphate into the pyrimidine synthesis pathway, resulting in increased orotic acid production.

The other options do not correlate directly with elevated serum ammonia in this scenario. High blood glucose is typical in conditions like diabetic ketoacidosis, which is not relevant here. Increased serum bilirubin might indicate hemolysis or liver dysfunction but is not specifically linked to elevated ammonia due to urea cycle disorders. Low serum bicarbonate may point to metabolic acidosis, which can occur in various conditions but is not directly a typical laboratory finding associated specifically with elevated ammonia. Therefore, elevated urine orotic acid is the most expected finding in this context.