## **Core Concept**
The diagnosis of diabetes mellitus is primarily based on the levels of blood glucose. The American Diabetes Association (ADA) provides specific guidelines for diagnosing diabetes through various tests, including fasting plasma glucose (FPG), oral glucose tolerance test (OGTT), and hemoglobin A1c (HbA1c) levels. These criteria help in identifying individuals with diabetes to initiate timely management and prevent complications.

## **Why the Correct Answer is Right**
The ADA guidelines specify that a diagnosis of diabetes can be made if the fasting plasma glucose (FPG) is 126 mg/dL or higher. This threshold is chosen because it is associated with an increased risk of diabetes-specific microvascular complications such as diabetic retinopathy. The diagnosis requires confirmation by repeating the test on a different day in the absence of unequivocal hyperglycemia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 100 mg/dL - This value corresponds to the impaired fasting glucose category, not diabetes. According to ADA guidelines, individuals with fasting glucose levels between 100 mg/dL and 125 mg/dL have impaired fasting glucose.
- **Option B:** 110 mg/dL - Although not listed in ADA criteria, values around this range might suggest prediabetes or impaired glucose regulation but do not meet the threshold for diabetes diagnosis.
- **Option D:** 200 mg/dL - This option might seem related to the random plasma glucose criterion for diabetes diagnosis (which is ≥200 mg/dL in the presence of symptoms), but it is not the threshold for fasting plasma glucose.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for the diagnosis of diabetes, the ADA criteria include:
- Fasting plasma glucose ≥126 mg/dL,
- 2-hour plasma glucose during OGTT ≥200 mg/dL,
- Random plasma glucose ≥200 mg/dL in the presence of hyperglycemic symptoms,
- HbA1c ≥6.5%.

## **Correct Answer:** . 126 mg/dL.