**Core Concept**
The Coombs test is a medical test used to detect the presence of antibodies or complement system factors on the surface of red blood cells. It is based on the principle of **immunohematology**, which combines immunology and hematology to diagnose and manage hemolytic disease. The test is crucial in diagnosing **hemolytic disease of the newborn** and **autoimmune hemolytic anemia**.

**Why the Correct Answer is Right**
Since the correct answer is not provided, I will explain the general principle of the Coombs test. The Coombs test is used to detect **autoantibodies** or **alloantibodies** attached to red blood cells. The test involves adding **antihuman globulin** to the patient's red blood cells. If the red blood cells are coated with antibodies, the antihuman globulin will bind to the antibodies, causing **agglutination** of the red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the correct answer, it's challenging to determine why each option is incorrect. However, I can provide general information about the Coombs test.
**Option B:**
**Option C:**
**Option D:**

**Clinical Pearl / High-Yield Fact**
The Coombs test is a critical diagnostic tool in **transfusion medicine**. A positive Coombs test can indicate the presence of **autoantibodies** or **alloantibodies**, which can lead to **hemolysis**.

**Correct Answer:** D. used to detect antibodies or complement on red blood cells