This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
An infant presented to the OPD with a history of vomiting and poor feeding. Musty odor is present in the baby's sweat and urine. Guthrie test was done and it was found to be positive. All are true regarding this disease except: 
Select and explain. 
A. Phenylacetate positive in urine, B. Tandem mass spectrometry is the gold standard test, C. Phenylalanine hydroxylase enzyme defect, D. mental retardation is absent
A: Phenylacetate positive in urine
Answer: A: Phenylacetate positive in urine 
Explanation: The given clinical condition is suggestive of phenylketonuria (PKU) PKU is caused by the deficiency of an enzyme phenylalanine hydroxylase. Guthrie bacterial inhibition test is a screening test for phenylketonuria. This test uses a phenylalanine requiring strain of Bacillus subtilis. Ferric chloride test is also used as a screening test for PKU. Tandem mass spectrometry is the gold standard investigation Reference: Harpers illustrated biochemistry 30th edition