**Core Concept:** C-peptide is a byproduct of the enzyme proinsulin-converting enzyme (PC1/3) when it cleaves proinsulin into insulin and C-peptide. Both insulin and C-peptide are released from the beta cells of the pancreas in response to raised blood glucose levels and play a role in blood glucose regulation.

**Why the Correct Answer is Right:** C-peptide is a 31-amino acid peptide that shares a common C-terminal sequence with insulin. It has no biological activity but is a reliable marker of beta-cell function. When blood glucose levels rise, beta cells secrete insulin and C-peptide simultaneously. The ratio of C-peptide to insulin reflects beta-cell function.

**Why Each Wrong Option is Incorrect:**

A. Insulin is the hormone responsible for lowering blood glucose levels. It is not the correct answer because C-peptide is a byproduct of insulin production and not a hormone itself.

B. Glucagon, another hormone secreted by alpha cells of the pancreas, raises blood glucose levels by stimulating gluconeogenesis and glycogenolysis. It is incorrect because C-peptide is a byproduct of insulin production, not a separate hormone.

C. Proinsulin is the precursor hormone for insulin. While proinsulin is processed into insulin and C-peptide, it is not the correct answer because proinsulin is the precursor, not a hormone itself.

D. Somatostatin is a hormone secreted by the D cells of the pancreas that inhibits the secretion of insulin and glucagon. It is incorrect because C-peptide is a byproduct of insulin production, not a separate hormone.

**Clinical Pearl:** C-peptide testing is an essential tool for evaluating beta-cell function in patients with type 1 and type 2 diabetes. Its levels can be used to diagnose latent autoimmune diabetes in adults (LADA) and monitor treatment response in patients with type 1 diabetes.