Which of the following medications primarily causes an increase in incretin levels?

Dipeptidyl peptidase-4 (DPP-4) inhibitors are medications that specifically target the incretin pathway of glucose metabolism. Incretins are hormones released from the gastrointestinal tract in response to food intake, and they play a crucial role in insulin secretion and blood glucose regulation.

DPP-4 is an enzyme that breaks down incretins like glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). By inhibiting DPP-4, these medications prolong the action of incretins, leading to increased levels of these hormones in the body. This results in enhanced insulin secretion, reduced glucagon release, and ultimately a decrease in blood sugar levels after meals.

In comparison, the other medication classes listed do not primarily operate through the incretin system. Thiazolidinediones primarily enhance insulin sensitivity; alpha-glucosidase inhibitors work by delaying carbohydrate absorption in the intestines; and meglitinides stimulate insulin secretion from the pancreas, but not through the incretin mechanism. Therefore, the primary role of DPP-4 inhibitors in increasing incretin levels makes them the correct choice for this question.