An infant presents to OPD with a history of vomiting and poor feeding. Musty odor is present in baby’s urine. Guthrie test was done and found to be positive. The most likely diagnosis is?
**Core Concept**
The Guthrie test is a bacterial inhibition assay used to diagnose phenylketonuria (PKU), an inborn error of metabolism characterized by the inability to metabolize the amino acid phenylalanine (Phe). This leads to its accumulation in the body, causing neurological damage and a musty odor in the urine.
**Why the Correct Answer is Right**
The Guthrie test detects the presence of phenylpyruvic acid (PPA) in the blood, which is a metabolite of phenylalanine. In PKU, the enzyme phenylalanine hydroxylase (PAH) is deficient, leading to the accumulation of PPA in the blood and its subsequent excretion in the urine. The musty odor in the urine is due to the presence of PPA. The Guthrie test is a simple and reliable method for screening newborns for PKU.
**Why Each Wrong Option is Incorrect**
**Option A:** Congenital hypothyroidism is a condition characterized by a deficiency of thyroid hormones, but it does not present with a musty odor in the urine.
**Option B:** Maple syrup urine disease is another inborn error of metabolism, but it presents with a sweet, maple syrup-like odor in the urine, not a musty odor.
**Option C:** Tyrosinemia type I is a condition characterized by the accumulation of tyrosine and its metabolites, but it does not present with a musty odor in the urine.
**Clinical Pearl / High-Yield Fact**
The Guthrie test is a classic example of a screening test that can prevent long-term neurological damage by early diagnosis and treatment of PKU. It is essential to remember that a positive Guthrie test requires confirmatory testing with phenylalanine levels to confirm the diagnosis.
**Correct Answer:** C.