13 - iced

[ArchivalUser]

A 47-year-old male patient presents with painful arthritis in the right big toe and uric acid renal stones. He has
been taking allopurinol for his condition. What biochemical defect would likely be found in this patient?


A. A defect in urea synthesis

B. An abnormality of the purine degradation pathway

C. An inability to synthesize non-essential amino acids

D. Defective topoisomerases

E. Increased levels of leukotrienes

[ArchivalUser]

The correct answer is B. This patient has gout, characterized by painful joints due to the precipitation of uric
acid crystals caused by excessive production of uric acid (a minority of cases are associated with
underexcretion of uric acid). Kidney disease is also seen due to accumulation of uric acid in the tubules. The
disease mostly affects males, and is frequently treated with allopurinol, an inhibitor of xanthine oxidase.
Xanthine oxidase catalyzes the sequential oxidation of hypoxanthine to xanthine to uric acid.

A defect in urea synthesis (choice A) would result in the accumulation of ammonia.

Phenylketonuria is a disease in which tyrosine cannot be produced from phenylalanine (choice C). It is
characterized by a musty body odor and mental retardation.

Defective topoisomerases (choice D) would affect DNA unwinding, and therefore replication.

Leukotrienes (choice E) are potent constrictors of smooth muscle and would more likely lead to
bronchoconstriction.