**Core Concept:** Oral Glucose Tolerance Test (OGTT) is a diagnostic test used to assess an individual's glucose tolerance, which indicates an individual's ability to respond to and utilize glucose. Impaired glucose tolerance (IGT) is a stage between normal glucose tolerance and type 2 diabetes mellitus, where the glucose levels are elevated but not as high as in diabetes.

**Why the Correct Answer is Right:** Impaired glucose tolerance is defined as having elevated blood glucose levels during an OGTT but not reaching the threshold for diabetes mellitus. In this case, the correct answer is C:

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because having elevated fasting blood glucose levels alone is not sufficient to diagnose impaired glucose tolerance. Elevated fasting blood glucose is a feature of prediabetes, not impaired glucose tolerance.
B. This option is incorrect as an impaired glucose tolerance diagnosis requires elevated blood glucose levels during the OGTT and not just after a meal or in the postprandial period.
D. This option is incorrect because having elevated blood glucose levels during the OGTT but normal fasting glucose levels does not meet the diagnostic criteria for impaired glucose tolerance.

**Why Option C is Right:** Elevated levels of blood glucose during the OGTT, along with normal fasting glucose levels, is indicative of impaired glucose tolerance, meeting the diagnostic criteria for this condition.

**Clinical Pearl:** OGTT is useful for diagnosing glucose intolerance and identifying individuals who may progress to develop type 2 diabetes mellitus. Early intervention can prevent or delay the onset of diabetes and its complications.