Question 1

Which of the following best describes the primary biochemical role of tetrahydrofolate ($$FH_4) in $$cellular metabolism?

Accepted Answer

Transfers one-carbon units in various oxidation states for biosynthetic reactions

Question 2

A patient presents with megaloblastic anemia. Laboratory tests reveal elevated homocysteine and methylmalonic acid. Which vitamin deficiency is most likely responsible?

Accepted Answer

Vitamin $$B_12$$

Question 3

Which enzyme catalyzes the conversion of 5,10-methylene-$$FH_4 to 5-$$methyl-$$FH_4$$, and what is the significance of this reaction?

Accepted Answer

Methylene-$$FH_4$$ reductase (MTHFR); it provides the methyl group for methionine synthesis

Question 4

Which of the following statements about folic acid and folate is CORRECT?

Accepted Answer

Folic acid must be reduced by dihydrofolate reductase (DHFR) to become biologically active

Question 5

Which reaction is directly inhibited by the cancer drug 5-fluorouracil (5-FU)?

Accepted Answer

Thymidylate synthase-mediated synthesis of dTMP from dUMP

Question 6

A deficiency in which enzyme would most likely result in the accumulation of S-adenosylhomocysteine (SAH) and decreased methylation capacity?

Accepted Answer

SAH hydrolase

Question 7

Which of the following best explains why rapidly dividing cells (e.g., bone marrow, GI epithelium) are most affected by folate deficiency?

Accepted Answer

They have a high demand for one-carbon units for DNA synthesis

Question 8

Which of the following is a direct consequence of methotrexate therapy?

Accepted Answer

Inhibition of the conversion of $$FH_2 to$$ $$FH_4$$

Question 9

Which vitamin is required as a cofactor for the conversion of homocysteine to cysteine in the transsulfuration pathway?

Accepted Answer

Vitamin $$B_6 ($$pyridoxal phosphate)

Question 10

A patient with a genetic deficiency in MTHFR (methylene-$$FH_4$$ reductase) is most likely to have impaired synthesis of which molecule?

Accepted Answer

5-methyl-$$FH_4$$

Question 11

Which of the following reactions is independent of both folate and vitamin $$B_12$$?

Accepted Answer

Methylation of homocysteine by betaine-homocysteine methyltransferase

Question 12

Which of the following best describes the methyl-trap hypothesis?

Accepted Answer

Vitamin $$B_12$$ deficiency causes accumulation of 5-methyl-$$FH_4$$ and functional folate deficiency

Question 13

Which of the following is a direct product of the reaction catalyzed by serine hydroxymethyltransferase (SHMT)?

Accepted Answer

5,10-methylene-$$FH_4$$

Question 14

Which of the following is NOT a function of S-adenosylmethionine (SAM)?

Accepted Answer

Directly donates formyl groups for purine synthesis

Question 15

Which of the following best explains why megaloblastic anemia is characterized by large, immature erythroid precursors?

Accepted Answer

Impaired DNA synthesis with continued cytoplasmic maturation

Question 16

Which of the following steps in purine biosynthesis requires a one-carbon unit from formyl-tetrahydrofolate (10-formyl-$$FH_4$$)?

Accepted Answer

Two separate steps in the de novo synthesis of AMP and GMP

Question 17

Which of the following is the correct sequence of events in the methylation cycle involving folate and vitamin $$B_12$$?

Accepted Answer

5-methyl-$$FH_4$$ donates a methyl group to vitamin $$B_12$$, which then transfers it to homocysteine to form methionine

Question 18

Which of the following is a distinguishing structural feature of vitamin $$B_12 ($$cobalamin)?

Accepted Answer

Contains a central cobalt atom in a corrin ring

Question 19

Which of the following best describes the clinical significance of elevated homocysteine levels?

Accepted Answer

Suggests impaired methylation and increased risk for cardiovascular disease

Question 20

Which vitamin is required for the conversion of methylmalonyl-CoA to succinyl-CoA, and what is the active cofactor form?

Accepted Answer

Vitamin $$B_12$$; adenosylcobalamin

Tetrahydrofolate (FH₄), Vitamin B₁₂, and S-Adenosylmethionine (SAM): Biochemical Roles in DNA Synthesis, Methylation, and Anemia

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