Deck 28: The Metabolism of Purines and Pyrimidines

Question 1

A 4-year-old girl presents in the clinic with megaloblastic anemia and failure to thrive.Orotic acid is demonstrated in blood and urine.Enzyme measurements of white blood cells reveal a deficiency of orotate phosphoribosyl transferase and abnormally high activity of aspartate transcarbamoylase.Which one of the following treatments will reverse all these symptoms?

A)Blood transfusion.
B)White blood cell transfusion.
C)Dietary supplements of phosphoribosylpyrophosphate.
D)Oral thymidine.
E)Oral uridine.

Answer 1

Oral uridine.

Question 2

The effect of the chemotherapeutic drug methotrexate is best described as:

A)Disruption of the mitotic spindle.
B)Impaired synthesis of deoxyribonucleotides from ribonucleotides.
C)Inhibition of orotic acid synthesis in the biosynthetic pathway of pyrimidine nucleotides.
D)Depletion of tetrahydrofolate.
E)Induction of mutations by DNA methylation.

Answer 2

Depletion of tetrahydrofolate.

Question 3

Allopurinol can cure gouty arthritis because it inhibits the enzyme:

A)Adenosine deaminase.
B)Thymidine kinase.
C)Ribonucleotide reductase.
D)Thymidylate synthase.
E)Xanthine oxidase.

Answer 3

Xanthine oxidase.

Question 4

Amethopterin (methotrexate) and fluorouracil are two drugs that are sometimes used in combination for cancer chemotherapy.Which of the following processes is inhibited by both of these drugs?

A)Reduction of ribonucleotides to deoxyribonucleotides.
B)Synthesis of purine nucleotides.
C)De novo synthesis of pyrimidine nucleotides.
D)Synthesis of thymine.
E)Polymerization of tubulin.

Question 5

A 1-year-old boy has a history of recurrent bacterial and fungal infections.Further testing shows increased concentrations of deoxyadenosine triphosphate (dATP) in white blood cells.Which of the following is the most likely diagnosis?

A)Adenosine deaminase deficiency.
B)Lesch-Nyhan syndrome.
C)Orotic aciduria.
D)Purine-nucleoside phosphorylase deficiency.
E)Xanthine oxidase deficiency.

Question 6

For cancer patients, folic acid deficiency can be a good thing because the cancer cells need folate coenzymes for rapid DNA replication.Which of the following cancer chemotherapeutic agents induces a deficiency of folate coenzymes?

A)Methotrexate.
B)Actinomycin D.
C)Asparaginase.
D)Azaserine.
E)Colchicine.

Question 7

A 1-year-old boy is brought to the pediatrician because his parents have noticed that he is mutilating himself by chewing on his lips.On examination, the child is found to be mentally retarded, and tophi are present in the earlobes.Which of the following enzymes is most likely deficient in this child?

A)Adenosine deaminase.
B)Hypoxanthine-guanine phosphoribosyltransferase.
C)Phosphoribosylpyrophosphate synthetase.
D)Xanthine oxidase.
E)Thymidylate kinase.

Question 8

Fluorouracil is a uracil analog used in the treatment of malignant disorders.It reacts covalently with which one of the following enzymes, causing irreversible inhibition?

A)Phosphoribosyl pyrophosphate aminotransferase.
B)Ribonucleotide reductase.
C)Thymidylate synthase.
D)Dihydrofolate reductase.
E)Adenosine deaminase.