**Question:** Diabetes mellitis diagnosed when

A. Random blood sugar above 200 mg/dL
B. Fasting blood sugar above 126 mg/dL
C. Postprandial blood sugar above 200 mg/dL
D. HbA1C above 6.5%

**Core Concept:** Diabetes mellitus is a group of chronic diseases characterized by high blood glucose levels due to defects in insulin secretion, insulin action, or both. This leads to long-term damage, dysfunction, and failure of various organs.

**Why the Correct Answer is Right:** Diabetes mellitus is diagnosed based on the American Diabetes Association (ADA) criteria. The Fasting Blood Sugar (FBS) test is a reliable method for diagnosing diabetes when performed using a standardized protocol, and the criteria for diagnosis are as follows:

1. Fasting Blood Sugar (FBS) above 126 mg/dL when the test is performed twice on separate days.
2. HbA1C above 6.5% when the test is performed twice on separate occasions.

**Why Each Wrong Option is Incorrect:**

A. Random Blood Sugar (RBS) above 200 mg/dL: While elevated RBS can indicate diabetes, the single value is not enough to diagnose diabetes. Two separate tests are required for diagnosis, as in option B and D.

C. Postprandial Blood Sugar (PBS) above 200 mg/dL: Similar to RBS, elevated PBS can indicate diabetes, but a single value is not enough for diagnosis. Two separate tests are required, as in option B and D.

**Clinical Pearl:**

Diagnosing diabetes involves a two-step process:
1. Measure Fasting Blood Sugar (FBS) or HbA1C levels, preferably after an 8-hour fasting period.
2. If the test result is abnormal, repeat the test on another day to confirm the diagnosis.

**Correct Answer:** B. Fasting Blood Sugar (FBS) above 126 mg/dL when the test is performed twice on separate days: This option correctly states the requirement of two separate tests to diagnose diabetes, as per the ADA criteria.