A 19-year-old male, at a routine physical examination for sports activities (long-distance running) at his college, is noticed to have elevated fasting blood glucose levels (about 7.5 mM). Measurements of C-peptide and insulin levels were close to normal under fasting conditions. After eating, blood glucose levels are only slightly elevated above the normal fasting levels before stabilizing at the fasting levels. The student indicates that he is not drinking or urinating excessively, but that he remembers that his mother had gestational diabetes when pregnant with him. This alteration in glucose homeostasis is best typified by which one of the following types of inheritance?

**Core Concept**
The patient in question exhibits a form of impaired glucose tolerance, characterized by an inability to adequately regulate blood glucose levels in response to meals, despite normal fasting glucose and insulin levels. This is suggestive of a condition known as **maturity-onset diabetes of the young (MODY)**, an autosomal dominant form of diabetes that typically presents in young adulthood.

**Why the Correct Answer is Right**
MODY is caused by mutations in genes that encode for transcription factors involved in pancreatic beta-cell function, such as _HNF1A_ or _HNF4A_. These mutations lead to impaired insulin secretion in response to meals, resulting in elevated postprandial glucose levels that normalize after fasting. The patient's family history of gestational diabetes may be a clue to the possibility of a genetic predisposition to diabetes, as gestational diabetes is a risk factor for developing MODY.

**Why Each Wrong Option is Incorrect**
**Option A:** **Type 1 diabetes** is an autoimmune condition characterized by absolute insulin deficiency, which is not consistent with the patient's normal C-peptide and insulin levels.
**Option B:** **Type 2 diabetes** is a condition of insulin resistance and impaired insulin secretion, typically presenting in older adults with obesity and other metabolic risk factors, which does not fit the patient's profile.
**Option C:** **Mitochondrial diabetes** is a rare form of diabetes caused by mutations in mitochondrial DNA, often presenting with a family history of diabetes and specific clinical features such as short stature and sensorineural hearing loss, which are not mentioned in the patient's history.

**Clinical Pearl / High-Yield Fact**
MODY is a common cause of diabetes in young adults, often presenting with a family history of diabetes, and is characterized by impaired insulin secretion in response to meals. It is essential to consider MODY in the differential diagnosis of young patients with impaired glucose tolerance, as it can have significant implications for treatment and management.

**Correct Answer: C. Mitochondrial diabetes**

✓ Correct Answer: A. Autosomal dominant