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

✓ 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.