**Core Concept**
Glycated haemoglobin (HbA1c) is a measure of the average blood glucose levels over the preceding 2-3 months. It reflects the degree of glycation of haemoglobin, which occurs when glucose molecules bind to the haemoglobin in red blood cells.

**Why the Correct Answer is Right**
The correct answer is that HbA1c reflects blood glucose levels over the preceding 2-3 months. This is because red blood cells have a lifespan of approximately 120 days, and during this time, they undergo continuous glycation. By measuring the amount of glycated haemoglobin in the blood, clinicians can estimate the average blood glucose levels over this period. This is particularly useful in the management of diabetes mellitus, as it provides a more accurate picture of glucose control over time.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HbA1c does not reflect blood glucose levels over the preceding few weeks.
**Option B:** This option is incorrect because HbA1c is not a measure of blood glucose levels in real-time.
**Option C:** This option is incorrect because HbA1c does not reflect blood glucose levels over the preceding few days.

**Clinical Pearl / High-Yield Fact**
It's essential to note that HbA1c levels can be affected by factors such as haemoglobinopathies, chronic kidney disease, and certain medications, which can lead to inaccurate results. Clinicians should interpret HbA1c levels in the context of the patient's overall clinical picture.

**Correct Answer:** 3. 2-3 months