Nucleotide Metab MCQ

Question 1

1. Inhibition of ribonucleotide reductase activity by dATP can result in
   A. Increased synthesis of purines
   B. Increased synthesis of pyrimidines
   C. Insufficient synthesis of dNTPs

Answer 1

C. Insufficient synthesis of dNTPs

Question 2

2. Ribonucleotide reductase activity is inhibited when:
   A. ATP is present in high quantities
   B. Ribonucleotides are present
   C. Adenosine deaminase is defective
   D. Hypoxanthine-guanine phosphoribosyl transferase is partially deficient
   E. Phosphoribosyl pyrophosphate synthase is active

Answer 2

C. Adenosine deaminase is defective

Question 3

3. A 4-year-old girl presents in the clinic with megaloblastic anemia and failure to thrive. Blood chemistries reveal orotic aciduria. Enzyme measurements of white blood cells reveal a deficiency of the pyrimidine biosynthesis enzyme orotate phosphoribosyltransferase and abnormally high activity of the enzyme aspartate transcarbamoylase. Which one of the following treatments will reverse all symptoms if carried out chronically?
   a. Blood transfusion
   b. White blood cell transfusion
   c. Dietary supplements of phosphoribosylpyrophosphate (PRPP)
   d. Oral thymidine
   e. Oral uridine

Answer 3

e. Oral uridine

Question 4

4. In most patients with gout as well as those with Lesch-Nyhan syndrome, purines are overproduced and overexcreted. Yet the hypoxanthine analogue allopurinol, which effectively treats gout, has no effect on the severe neurological symptoms of Lesch-Nyhan patients because it does not
   a. Decrease de novo purine synthesis
   b. Decrease de novo pyrimidine synthesis
   c. Diminish urate synthesis
   d. Increase phosphoribosylpyrophosphate (PRPP) levels
   e. Inhibit xanthine oxidase

Answer 4

a. Decrease de novo purine synthesis

Question 5

5. Which of the following would rule out hyperuricemia (high levels of uric acid in blood) in a patient?
   a. Lesch-Nyhan syndrome
   b. Gout
   c. Xanthine oxidase hyperactivity
   d. Carbamoyl phosphate synthase deficiency
   e. Purine overproduction secondary to Von Gierke’s disease

Answer 5

d. Carbamoyl phosphate synthase deficiency

Question 6

6. Which one of the following contributes nitrogen atoms to both purine and pyrimidine rings?
   a. Aspartate
   b. Carbamoyl phosphate
   c. Carbon dioxide

Answer 6

a. Aspartate

Question 7

7. Which statement best describes xanthine?
   a. It is a direct precursor of guanine
   b. It covalently binds to allopurinol
   c. It is a substrate rather than a product of the enzyme xanthine oxidase
   d. It is oxidized to form uric acid

Answer 7

d. It is oxidized to form uric acid

Question 8

8. Feedback inhibition of pyrimidine nucleotide synthesis can occur by which of the following means?
   a. Increased activity of carbamoyl phosphate synthetase
   b. Increased activity of aspartate transcarbamoylase
   c. CTP allosteric effects
   d. UMP competitive inhibition

Answer 8

c. CTP allosteric effects

Question 9

9. Purine nucleotide biosynthesis can be inhibited by which of the following?
   a. Guanosine triphosphate (GTP)
   b. Uridine monophosphate (UMP)
   c. Adenosine monophosphate (AMP)
   d. Adenosine triphosphate (ATP)
   e. Inosine diphosphate (IDP)

Answer 9

c. Adenosine monophosphate (AMP)

Question 10

10. Which base derivative can serve as a precursor for the synthesis of
two of the other base derivatives shown?
a. Cytidine triphosphate (CTP)
b. Uridine monophosphate (UMP)
c. Deoxythymidine monophosphate (dTMP)
d. Adenosine triphosphate (ATP)
e. Deoxyadenosine monophosphate (dAMP)

Answer 10

b. Uridine monophosphate (UMP)

Question 11

11. Which of the following compounds is a required substrate for purine biosynthesis?
    a. 5-methyl thymidine
    b. AraC
    c. Ribose phosphate
    d. 5-phosphoribosylpyrophosphate (PRPP)
    e. 5-fluorouracil

Answer 11

c. Ribose phosphate

Question 12

12. Which of the following statements about gout (A disease in which defective metabolism of uric acid causes arthritis) is incorrect?
    A. There is impaired excretion of uric acid
    B. There is increased production of uric acid
    C. Allopurinol treatment will result in a build up of hypoxanthine
    D. Allopurinol will result in accumulation of IMP and XMP
    E. Allopurinol treatment will result in excretion of xanthine and hypoxanthine

Answer 12

C. Allopurinol treatment will result in a build up of hypoxanthine

Question 13

13. Which of the following compounds is an analogue of hypoxanthine?
    a. AraC
    b. Allopurinol
    c. Ribose phosphate
    d. 5-phosphoribosylpyrophosphate

Answer 13

b. Allopurinol