## **Core Concept**
The diagnosis of overt diabetes mellitus is primarily based on the levels of blood glucose. Fasting blood glucose levels are one of the key diagnostic criteria. According to the American Diabetes Association (ADA) and other international guidelines, specific thresholds for fasting blood glucose are used to diagnose diabetes.

## **Why the Correct Answer is Right**
The correct answer, $geq 126$ mg/dL, is based on the ADA's criteria for the diagnosis of diabetes mellitus. A fasting plasma glucose (FPG) level of 126 mg/dL or higher is indicative of overt diabetes. This threshold is used because it reflects a level of glycemia that is associated with the microvascular complications of diabetes, such as diabetic retinopathy and nephropathy. The World Health Organization (WHO) also endorses similar criteria.

## **Why Each Wrong Option is Incorrect**
- **Option A:** $<100$ mg/dL is considered normal fasting glucose levels. Values in this range do not indicate diabetes. - **Option B:** $100-125$ mg/dL is classified as impaired fasting glucose (pre-diabetes), indicating a higher risk of developing diabetes but not diagnostic of overt diabetes. - **Option D:** $geq 200$ mg/dL is a criterion for the diagnosis of diabetes but is based on a random plasma glucose level or a 2-hour plasma glucose level during an oral glucose tolerance test (OGTT), not fasting blood glucose. ## **Clinical Pearl / High-Yield Fact** A key point to remember is that for the diagnosis of diabetes, the test must be confirmed on a subsequent day in the absence of unequivocal hyperglycemia. This means if a patient has a fasting glucose of $geq 126$ mg/dL on one occasion, it should be repeated to confirm the diagnosis, except in cases of clear symptoms of hyperglycemia. ## **Correct Answer:** . $geq 126$ mg/dL