Endocrine Pharmacology USMLE Step 1 Practice Question

A 62-year-old woman with a 10-year history of type 2 diabetes mellitus comes to the clinic because of progressive fatigue, generalized weakness, and numbness of her feet over the past 6 months. She has been taking the same oral antidiabetic medication for several years and reports good medication adherence. She denies alcohol use and follows a balanced diet. Her BMI is 31 kg/m². Vital signs are within normal limits. Physical examination reveals decreased vibratory sensation in the bilateral lower extremities. Laboratory studies show a hemoglobin of 10.2 g/dL, MCV of 104 fL, macrocytosis on peripheral blood smear, and a low serum vitamin B12 level. Which of the following drugs is most likely responsible?