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CASE FILES: BIOCHEMISTRY
C O M P R E H E N SIO N Q U E ST IO N S
[38.1] A 3-month-old boy presents with elevated levels of phenylalanine,
para-hydroxyphenylpyruvate and phenylpyruvate in the serum. His
skin color is pale. On your differential diagnosis is phenylketonuria.
Which of the following would be consistent in such a case?
A. Elevated levels of homogentisic acid in the serum
B. A deficiency in vitamin B
12
(cobalamin)
C. Elevated levels of pyridoxal phosphate in the serum
D. The urine in the boy’s diaper smells like fresh maple syrup
E. Phenylalanine hydroxylase activity is only 2 percent of normal
[38.2] A 1-year-old girl presents at your clinic the day after you saw the 3-
month-old boy. The symptoms are the same so you order a test on
phenylalanine hydroxylase to confirm your diagnosis of phenylke-
tonuria. To your surprise the phenylalanine hydroxylase activity is well
within the normal range. Which of the following might you check next
to support your diagnosis?
A. Tyrosine: a-ketoglutarate transaminase
B. Tyrosinase
C. Homogentisic acid oxidase
D. Dihydropteridine reductase
E. Dopamine hydroxylase
[38.3] Skin color is the aggregate result of the expression of a number of genes
modified by ethnic origin and genetic inheritance. Hypopigmentation
may be caused by which of the following?
A. Excess formation of melanin
B. Excess phenylalanine in the serum and tissues
C. Hyposecretion of melatonin
D. Excessive stimulation of tyrosinase
E. Low levels of para-hydroxyphenylpyruvate
A nsw ers
[38.1]
E. The correct response is very low levels of phenylalanine hydroxylase,
a key enzyme in the metabolic sequelae of phenylketonuria, that is, ele-
vated phenylalanine, phenylpyruvate, and
para
-hydroxyphenylpyruvate
in blood. Homogentisic acid is an intermediate in the breakdown of
tyrosine to fumarate and acetoacetate. Vitamin B
12
is required in the
metabolism of branched-chain amino acids not phenylalanine. The
a-keto acids of the branched chain amino acids produce the maple-
syrup odor.
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