Free Step 1-style question
Musty odor + fair complexion + developmental delay
A 10-month-old boy with fair skin, light-colored eyes, and a musty body odor is found to have a serum phenylalanine concentration of 32 mg/dL. He was recently diagnosed with phenylketonuria.
Which of the following substances is most likely to be significantly decreased in this patient?
- A. Homogentisic acid
- B. Melanin
- C. Orotic acid
- D. Phenylpyruvate
- E. Propionyl-CoA
Correct answer: B. Melanin
In classic phenylketonuria, phenylalanine hydroxylase deficiency prevents conversion of phenylalanine to tyrosine. Tyrosine is normally a precursor for melanin synthesis. When tyrosine availability is reduced, melanin production decreases, contributing to the fair skin and light-colored eyes seen in affected patients. Phenylalanine and phenylketones increase, but downstream products dependent on tyrosine availability, such as melanin, are decreased.
Takeaway
Classic PKU decreases tyrosine production from phenylalanine. Because tyrosine is a melanin precursor, affected patients can have reduced melanin synthesis with fair complexion and light-colored eyes.
What this page covers
Practice Step 1-style biochemistry questions on Classic PKU (phenylalanine hydroxylase deficiency), including Musty odor, fair complexion, developmental delay, with emphasis on metabolic consequence / accumulation / deficiency and answer-choice reasoning.
Step 1 practice focus
This preview is organized around Classic PKU (phenylalanine hydroxylase deficiency) in Phenylalanine Metabolism Disorders within Amino Acid Metabolism. It is intended for students practicing metabolic consequence / accumulation / deficiency questions, where the goal is to connect the vignette clue pattern to the underlying biochemical pathway, enzyme defect, metabolite change, regulatory step, or physiologic consequence.
How to use this page
Review the topic and reasoning focus, then practice Step 1-style questions inside BiochemStep. The question set emphasizes mechanism-first answer-choice reasoning rather than passive content review.