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The parents of a 6-month-old child who was normal at birth bring her into the clinic. Since their emigration to the
U.S. from Eastern Europe soon after her birth, the child has developed diminished responsiveness, progressive
blindness and deafness, and recently, seizures. Serum levels of which of the following compounds would be
expected to be decreased in both of the parents?
A. Dystrophin
B. Hexosaminidase A
C. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
D. Phenylalanine hydroxylase
E. Vitamin D3
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yes
The correct answer is B. This patient has Tay-Sachs disease, an autosomal recessive disorder caused by the
deficiency of hexosaminidase A, which leads to the accumulation of ganglioside GM2 in neurons, producing a
degenerative neurologic disease. Children appear normal at birth, but then begin to suffer from diminished
responsiveness, deafness, blindness, loss of neurologic function, and seizures. A cherry-red spot on the macula
may be seen by ophthalmoscopic examination. Death usually occurs by 4 to 5 years of age. There is no therapy.
The incidence is higher among Jews of Eastern European descent. Since the parents must be heterozygotes for
the mutant hexosaminidase A allele, they would be expected to have diminished levels of the enzyme.
A defect in the dystrophin (choice A) gene produces Duchenne muscular dystrophy, characterized by onset of
weakness in early childhood.
A severe deficiency in HGPRT (choice C) will lead to Lesch-Nyhan syndrome, characterized by excessive uric
acid production, mental retardation, spasticity, self-mutilation, and aggressive, destructive behavior.
Deficiency of phenylalanine hydroxylase (choice D) results in classic phenylketonuria, a disease in which
phenylalanine, phenylpyruvate, phenylacetate, and phenyllactate accumulate in plasma and urine. Clinically,
there is a musty body odor and mental retardation.
Hypophosphatemic rickets is an X-linked dominant condition causing abnormal regulation of vitamin D3 (choice
E) metabolism and defects in renal tubular phosphate transport. Symptoms include growth retardation,
osteomalacia, and rickets.
