You are an ER physician examining a 67-year-old
Woman. She was brought to the hospital by her neighbor because he found her passed out in the front yard. History reveals that she lives by herself and consumes a diet which consists primarily of lettuce and rice. She has not consumed any meat products for over 5 years and does not supplement her diet with vitamins. She reports that she is frequently quite tired. Blood work indicates she has severe megaloblastic anemia. Which of the following reaction pathways is most likely impaired in this patient?

Question

Quizplus · Accepted Answer

The patient's diet and symptoms suggest a vitamin B12 or folate deficiency, which impairs thymidine nucleotide biosynthesis, leading to megaloblastic anemia. This patient is suffering from a dietary deficiency in vitamin B₁₂. A deficiency in B₁₂ leads to impairment in the methionine synthase-catalyzed reaction leading to trapping of folate as N⁵-methyltetrahydrofolate. This trapping prevents the synthesis of other THF derivatives required for the purine and thymidine nucleotide biosynthesis pathways. The inability to synthesize DNA during erythrocyte maturation leads to abnormally large erythrocytes termed macrocytic or megaloblastic erythrocytes.

You Are an ER Physician Examining a 67-Year-Old