**Core Concept**
Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia due to defects in insulin secretion, insulin action, or both. Accurate diagnosis and monitoring of diabetes are crucial for managing the condition and preventing complications. Various tests have been developed to diagnose and monitor diabetes mellitus.

**Why the Correct Answer is Right**
To determine which option is not a test for diabetes mellitus, let's examine each choice. **Option A** is the Fasting Plasma Glucose (FPG) test, which measures the level of glucose in the blood after an overnight fast and is a standard test for diagnosing diabetes mellitus. **Option B** is the Oral Glucose Tolerance Test (OGTT), which evaluates the body's ability to regulate blood sugar levels after consuming a sugary drink. **Option C** is the Hemoglobin A1c (HbA1c) test, which measures the average blood glucose levels over the past 2-3 months by assessing the percentage of hemoglobin that has been glycated. **Option D** is the Complete Blood Count (CBC), which is a comprehensive test that measures various components of the blood, including red and white blood cells, platelets, and hemoglobin, but is not directly related to diabetes mellitus diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** While FPG is a test for diabetes mellitus, it's not the only option listed. However, this option is not the one to be ruled out.
**Option B:** OGTT is indeed a test for diabetes mellitus, particularly for diagnosing gestational diabetes or impaired glucose tolerance.
**Option C:** HbA1c is also a test for diabetes mellitus, widely used for monitoring blood sugar control in individuals with diabetes.
**Option D:** CBC is a test that measures various components of the blood, but it is not used to diagnose diabetes mellitus.

**Clinical Pearl / High-Yield Fact**
When diagnosing diabetes mellitus, it's essential to consider the patient's symptoms, medical history, and laboratory results. Fasting plasma glucose, OGTT, and HbA1c are essential tests for diagnosing diabetes mellitus, while CBC is a more general test that measures various blood components.

**Correct Answer: D. CBC is not a test for diabetes mellitus.**