**Core Concept**
Phenylpyruvic acid is a metabolite of phenylalanine, an essential amino acid. Its presence in the urine is a hallmark of phenylketonuria (PKU), a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase. The detection of phenylpyruvic acid in urine is crucial for diagnosing PKU.

**Why the Correct Answer is Right**
The ferric chloride test is a chemical test used to detect the presence of phenylpyruvic acid in urine. When ferric chloride is added to a urine sample containing phenylpyruvic acid, it forms a purple or blue color due to the formation of a complex between the ferric ion and the phenylpyruvic acid. This reaction is specific to phenylpyruvic acid and is not seen with other metabolites. The ferric chloride test is a simple and rapid method for screening for PKU.

**Why Each Wrong Option is Incorrect**
* **Option A:** Guthrie's test is a bacterial inhibition assay used to detect phenylalanine in blood, not phenylpyruvic acid in urine.
* **Option B:** VMA (vanillylmandelic acid) is a metabolite of catecholamines, such as adrenaline and noradrenaline, and is detected by a different chemical test.
* **Option C:** There is no such test as the "Gerhas test" for detecting phenylpyruvic acid in urine.

**Clinical Pearl / High-Yield Fact**
The ferric chloride test is a useful screening tool for PKU, but it is not definitive. A positive result requires confirmation with a more specific test, such as a bacterial inhibition assay or gas chromatography-mass spectrometry (GC-MS).

**✓ Correct Answer: D. Ferric chloride test**