<prompt> For differentiating between Insulinoma and Sulfonylurea related hypoglycemia, the test which is useful is ?Antibody to InsulinPlasma C-peptide levelPlasma Insulin levelInsulin: Glucose ratio  <response>: Surreptitious use of insulin or hypoglycemic agents like sulfonylureas may be difficult to distinguish from insulinomas. The combination of proinsulin levels normal in exogenous insulinhypoglycemic agent users, C-peptide levels low in exogenous insulin users, antibodies to insulin positive in exogenous insulin users, and measurement of sulfonylurea levels in serum or plasma will allow the correct diagnosis to be made. Reference : page 569 Harrisons Principles of Internal Medicine 19th edition 