**Core Concept**
Glucose tolerance test (GTT) is a diagnostic tool used to assess the body's ability to regulate blood glucose levels after consuming a sugary drink. The test evaluates the body's insulin sensitivity and pancreatic function by measuring blood glucose levels at specific intervals.

**Why the Correct Answer is Right**
During a standard oral glucose tolerance test, a patient is asked to fast for at least 8 hours before consuming a sugary drink containing 75 grams of glucose. Blood glucose levels are then measured at 1, 2, and 3 hours after consumption. This procedure helps assess the patient's ability to regulate blood glucose levels and can diagnose conditions such as diabetes mellitus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the fasting period is less than 8 hours, which can lead to inaccurate results.
**Option B:** This option is incorrect as the glucose load is less than 75 grams, which may not be sufficient to assess the body's glucose regulation.
**Option C:** This option is incorrect as the blood glucose levels are only measured at 1 hour, which is not sufficient to assess the body's glucose regulation over time.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) recommends a 75-gram glucose load for the oral glucose tolerance test, and blood glucose levels should be measured at 1, 2, and 3 hours after consumption to accurately diagnose diabetes mellitus.

**Correct Answer: D. The patient is asked to fast for at least 8 hours before consuming a sugary drink containing 75 grams of glucose, and blood glucose levels are then measured at 1, 2, and 3 hours after consumption.