**Core Concept**
The underlying principle being tested is a genetic disorder affecting an enzyme crucial for the metabolism of a specific amino acid, leading to its accumulation in the body and causing characteristic clinical manifestations.

**Why the Correct Answer is Right**
The disorder described is due to a deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD). The reducing substance in the urine is likely glucose, which is present due to the body's attempt to break down the accumulated amino acid, phenylalanine, in the process of gluconeogenesis. The test with ferric chloride yields a positive result due to the presence of phenylpyruvic acid, a metabolite of phenylalanine. The deficiency of G6PD leads to an inability to protect red blood cells from oxidative damage, which is exacerbated by certain medications and infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the disorder described does not involve a deficiency of glucose-6-phosphate dehydrogenase (G6PD), even though it is related to a metabolic disorder.
* **Option B:** This option is incorrect because the disorder described does not involve a deficiency of galactose-1-phosphate uridyltransferase (GALT), which is related to galactosemia.
* **Option C:** This option is incorrect because the disorder described does not involve a deficiency of fructose-1,6-bisphosphatase (FBPase-1), which is related to hereditary fructose intolerance.

**Clinical Pearl / High-Yield Fact**
Infants with phenylketonuria (PKU) may have a "musty" odor to their breath and sweat due to the accumulation of phenylalanine metabolites.

**Correct Answer: D. Fructose-1,6-bisphosphatase (FBPase-1)**