This question was asked in a medicine quiz I was participating in.

An 8-day-old boy is brought to the physician by his mother because of vomiting and poor feeding. The pregnancy was uncomplicated, and he was born at full term. He appears pale and lethargic. Physical examination shows diffusely increased muscle tone. His urine is noted to have a sweet odor. This patient's symptoms are most likely caused by the accumulation of which of the following:

A. Isoleucine B. Homocysteine C. Homogentisic acid D. Phenylalanine

As a first year med student, I am most probably wrong, but please tell me where-

The accumulation of homocystine, i.e. homocystinurea is mostly related to poor vision and osteoporosis in children. So that can be eliminated here.

HMG acid accumulation, leading to alkaptonurea typically presents in adults, and is relatively mild, so that can be eliminated too. Also, urine turning black is nowhere mentioned here.

What I can conclude, is that PKU (due to phenylalanine accumulation) and MSUD (due to branched chain amino acid-- isoleucine accumulation) are both possible. Because both of these conditions present with 1. Increased muscle tone, and 2. Sweet odor in urine. I can't figure out why, the answer was given as A. Isoleucine.

How can I eliminate PKU in this case?

(PKU- Phenylketonurea MSUD- Maple syrup urine disease)

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