A nursing student administered a biguanide medication to a client with Type 2 diabetes. Which of the following oral diabetes agents is classified as a biguanide?

Rationale: A. Metformin is the only biguanide commonly used in Type 2 diabetes. It works by decreasing hepatic glucose production, improving insulin sensitivity, and enhancing peripheral glucose uptake, without directly stimulating insulin secretion. It is first-line therapy for most patients with Type 2 diabetes. B. Sitagliptin is a DPP-4 inhibitor. It works by enhancing incretin hormones, which increases insulin release and decreases glucagon levels in a glucose-dependent manner. It is not a biguanide and does not primarily affect hepatic glucose production. C. Glipizide is a sulfonylurea. It lowers blood glucose by stimulating insulin secretion from pancreatic beta cells, which differs mechanistically from biguanides. Using glipizide does not classify as administering a biguanide. D. Pioglitazone is a thiazolidinedione (TZD). It works by increasing insulin sensitivity in muscle and adipose tissue through activation of PPAR-gamma receptors. Although it improves glucose utilization, it is not a biguanide and does not reduce hepatic gluconeogenesis like metformin.