**Core Concept**
Type 2 diabetes mellitus (T2DM) is a complex disorder influenced by both genetic and environmental factors, often presenting in adulthood with impaired insulin sensitivity and insulin secretion. The risk of developing T2DM increases with family history, obesity, and age.

**Why the Correct Answer is Right**
Given the patient's young age, normal BMI, and absence of ketonuria, it suggests that he may not have an underlying defect in insulin production or a severe impairment in insulin sensitivity. His family history is also suggestive of a possible genetic predisposition to T2DM, particularly since his father, who is the only son of his diabetic grandfather, did not develop the disease. This pattern is consistent with the autosomal dominant inheritance of MODY (Maturity-Onset Diabetes of the Young), a subtype of T2DM.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll ignore it.
**Option B:** This option is not provided, so we'll ignore it.
**Option C:** This option is not provided, so we'll ignore it.
**Option D:** This option is not provided, so we'll ignore it.

**Clinical Pearl / High-Yield Fact**
MODY is a group of rare, autosomal dominant genetic disorders characterized by early-onset diabetes, often before the age of 25. It's essential to consider MODY in young patients with a positive family history of diabetes, as they may require a different treatment approach.

**Correct Answer:** A.