A newborn screens positive for phenylketonuria (PKU). This condition is primarily caused by a deficiency in which enzyme?

A Tyrosinase

B Phenylalanine hydroxylase

C Homogentisate oxidase

D Alpha-ketoacid dehydrogenase

✓ Correct Answer: Option B

PKU is an autosomal recessive disorder caused by a deficiency in phenylalanine hydroxylase. This enzyme converts phenylalanine to tyrosine. Its deficiency leads to the accumulation of phenylalanine and its toxic byproducts.

💡 Why is B) the correct answer?

The correct answer to the question "A newborn screens positive for phenylketonuria (PKU). This condition is primarily caused by a deficiency in which enzyme?" is Option B: Phenylalanine hydroxylase.

The other options — A) Tyrosinase, C) Homogentisate oxidase, D) Alpha-ketoacid dehydrogenase — are incorrect for this USMLE Exam Prep question. As explained above: PKU is an autosomal recessive disorder caused by a deficiency in phenylalanine hydroxylase. This enzyme converts phenylalanine to tyrosine. Its deficiency leads to the accumulation of phenylalanine a…

📚 About This USMLE Exam Prep Practice Question

This 2-level USMLE Exam Prep question from the Step 1 – Basic Sciences topic is part of BrainForge Quiz's free exam preparation library. Each question includes the correct answer and a detailed explanation to help you understand the underlying concept, not just memorize the answer.

How to use this page: Read the question above, consider all four options (A through D), then check the highlighted correct answer and explanation. For active timed practice, start a free USMLE Exam Prep practice test or use flashcards for spaced repetition review.

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A newborn screens positive for phenylketonuria (PKU). This condition is primarily caused by a deficiency in which enzyme?

✓ Correct Answer: Option B

📚 About This USMLE Exam Prep Practice Question

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