A 50-year-old man comes to the physician because of a 3-week history of increased fatigue. He has a history of crohn disease. One year ago, he has admitted for lower abdominal pain and underwent for terminal ilium resection. Examination shows pallor of the mucosal membranes, mild scleral icterus, a swollen, red tongue, and several mouth ulcers. There is darkening of the skin over the dorsal surfaces of the fingers, toes, and creases of the palms and soles. The abdomen is soft and nontender; there is no palpable hepatosplenomegaly. Laboratory studies show a hemoglobin concentration of 9.1 gm/dL, Peripheral blood smear is shown below: Which of the following is most likely responsible for this patient's current condition?

Question

A 50-year-old man comes to the physician because of a 3-week history of increased fatigue. He has a history of crohn disease. One year ago, he has admitted for lower abdominal pain and underwent for terminal ilium resection. Examination shows pallor of the mucosal membranes, mild scleral icterus, a swollen, red tongue, and several mouth ulcers. There is darkening of the skin over the dorsal surfaces of the fingers, toes, and creases of the palms and soles. The abdomen is soft and nontender; there is no palpable hepatosplenomegaly.  Laboratory studies show a hemoglobin concentration of 9.1 gm/dL, Peripheral blood smear is shown below:  Which of the following is most likely responsible for this patient's current condition?

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Eductional Objective: Vitamin B12 is an essential cofactor in the recycling of 5-methyl-tetrahydrofolate to tetrahydrofolate, which is required for amino acid, purine, and thymidine synthesis. Deficiency primarily affects hematopoietic cell maturation and is marked by increased nuclear-cytoplasmic ratio, leading to macrocytic red blood cells and hypersegmented neutrophils; increased apoptosis often also leads to anemia and mild thrombocytopenia/leukopenia.

A 50-Year-Old Man Comes to the Physician Because of a 3-Week