A 6-month-old infant from rural Maharashtra presents with developmental delay, light skin, blonde hair, and a musty odor in the urine. Newborn screening was not performed. Regarding the biochemical and clinical features of this condition, all of the following are true EXCEPT:

Question

Accepted Answer

A. The musty odor is due to accumulation of phenylacetate and phenylacetyl-CoA in urine and sweat. ## Musty Odor in Phenylketonuria (PKU)

**Key Point:** The characteristic musty/mousy odor in PKU is due to **phenylacetate** (and to a lesser extent phenylpyruvate) excreted in urine and sweat — **NOT phenylacetyl-CoA**, which is an intracellular metabolic intermediate and is not excreted.

### Why Option A is FALSE (the EXCEPT answer)

The metabolic pathway in PKU:

1. Phenylalanine → **Phenylpyruvate** (transamination)
2. Phenylpyruvate → **Phenylacetate** (decarboxylation)
3. Phenylpyruvate → **Phenyllactate** (reduction)

**Phenylacetate** is the volatile compound responsible for the characteristic musty/mousy odor in urine and sweat. **Phenylacetyl-CoA** is an activated intermediate formed intracellularly (used in conjugation reactions, e.g., with glutamine to form phenylacetylglutamine) — it is NOT excreted and does NOT contribute to the odor. Including phenylacetyl-CoA as a cause of the musty odor is factually incorrect.

*(Reference: Harper's Illustrated Biochemistry, 31st ed.; Lehninger Principles of Biochemistry)*

### Why Other Options Are Correct (TRUE statements)

| Option | Statement | Validity |
|--------|-----------|----------|
| B | Early dietary phenylalanine restriction prevents intellectual disability | ✓ Correct — diet started within 2–4 weeks of life prevents neurological damage; delay beyond 3 months causes permanent intellectual disability |
| C | Light skin/blonde hair from inhibition of tyrosinase by elevated phenylalanine reducing melanin synthesis | ✓ Correct — phenylalanine and its metabolites competitively inhibit tyrosinase AND compete with tyrosine at neutral amino acid transporters, reducing melanin synthesis; this is an accepted and standard explanation in textbooks |
| D | Plasma Phe >20 mg/dL (>1200 µmol/L) is diagnostic; autosomal recessive inheritance | ✓ Correct — classic PKU threshold is >20 mg/dL; caused by homozygous PAH gene mutations; autosomal recessive |

**High-Yield:** The musty odor in PKU = **phenylacetate** (excreted). Phenylacetyl-CoA is an intracellular intermediate used in conjugation — it is NOT the odor-causing compound. This is a classic NEET PG discriminator.

**Clinical Pearl:** PKU is the prototype inborn error of amino acid metabolism. Newborn screening (Guthrie test / tandem mass spectrometry) detects elevated phenylalanine within 48–72 hours of birth, enabling early dietary intervention that prevents intellectual disability entirely.

Answer

B. The light skin and blonde hair result from inhibition of tyrosinase by elevated phenylalanine, reducing melanin synthesis

Answer

C. Intellectual disability in this condition is primarily preventable by early dietary restriction of phenylalanine starting within the first few weeks of life

Answer

D. Elevated plasma phenylalanine levels (>20 mg/dL or >1200 µmol/L) are diagnostic, and the condition follows an autosomal recessive inheritance pattern

A 6-month-old infant from rural Maharashtra presents with developmental delay, light skin, blonde hair, and a musty odor in the urine. Newborn screening was not performed. Regarding the biochemical and clinical features of this condition, all of the following are true EXCEPT:

Explanation

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