**Core Concept**
Long-term control of diabetes is assessed by measuring glycemic variability and its impact on microvascular and macrovascular complications. Glycated hemoglobin (HbA1c) is a key marker that reflects the average blood glucose levels over the past 2-3 months.

**Why the Correct Answer is Right**
Glycated hemoglobin (HbA1c) is formed when glucose binds to hemoglobin in red blood cells. The HbA1c level is directly proportional to the average blood glucose concentration. It is a widely accepted standard for assessing long-term glycemic control, as it provides a more accurate picture of glucose levels over time compared to a single blood glucose measurement. The American Diabetes Association recommends HbA1c as a primary tool for assessing diabetes control.

**Why Each Wrong Option is Incorrect**
**Option A:** Fasting glucose is a useful measure for diagnosing diabetes but does not accurately reflect long-term glycemic control.
**Option B:** Urine glucose testing is not a reliable measure of glycemic control, as it can be affected by various factors such as renal function and hydration status.
**Option C:** Oral glucose tolerance test (OGTT) is used to diagnose diabetes and prediabetes but is not a suitable measure for long-term control.

**Clinical Pearl / High-Yield Fact**
Remember that HbA1c levels can be affected by factors such as anemia, hemoglobinopathies, and certain medications, so it is essential to interpret results in the context of the patient's clinical presentation.

**Correct Answer: A. Fasting glucose is a useful measure for diagnosing diabetes but does not accurately reflect long-term glycemic control.**