This patient's symptoms, along with lab findings of hyperglycemia and glucosuria, suggest diabetes mellitus. The absence of GAD antibodies and low C-peptide levels point to type 1 diabetes mellitus. GAD antibodies, when present, suggest an autoimmune cause, as GAD is an enzyme targeted by autoimmune antibodies in type 1 diabetes. Low C-peptide levels indicate reduced endogenous insulin production, another hallmark of type 1 diabetes (as it is due to pancreatic beta cell destruction).

Most common in obese individuals and usually presents later in life. Insulin resistance and increased endogenous insulin production (suggested by high C-peptide levels) are characteristic features.

Occurs during pregnancy due to placental hormones causing insulin resistance. It would be less likely in this case due to the absence of pregnancy and the low C-peptide levels indicating reduced insulin production.

A form of monogenic diabetes that often presents in adolescence or early adulthood, but MODY typically does not result in decreased C-peptide levels.