This drug works as a competitive reversible inhibitor of DPP-4, reducing enzymatic inactivation of the incretin hormone, GLP-1.

• A. Metformin
• B. Pioglitazone
• C. Nateglinide
• D. Sitagliptin

Answer: (D)

This describes an incretin-based mechanism: the drug inhibits DPP-4, the enzyme that breaks down GLP-1. By blocking DPP-4 in a competitive, reversible way, levels of GLP-1 (and other incretins) stay higher after meals. GLP-1 boosts insulin release from pancreatic beta cells in a glucose-dependent manner and lowers glucagon secretion from alpha cells, which together improves postprandial glucose control without driving insulin release when glucose isn’t elevated. Sitagliptin fits this profile as a selective DPP-4 inhibitor, leading to prolonged active incretin action.

The other drugs act through different paths. Metformin lowers hepatic glucose production; pioglitazone improves insulin sensitivity via PPAR gamma activation; nateglinide stimulates insulin release by closing pancreatic KATP channels but does not affect DPP-4 or incretins.