## Core Concept
The question pertains to the diagnosis of **beta-thalassemia trait**, also known as beta-thalassemia minor. Beta-thalassemia is a genetic disorder affecting hemoglobin synthesis, characterized by reduced production of the beta-globin chains of hemoglobin. The beta-thalassemia trait is a heterozygous state, where one of the two beta-globin genes is mutated, leading to mild anemia.

## Why the Correct Answer is Right
The correct answer involves understanding that the **Complete Blood Count (CBC)** and **peripheral blood smear** are initial tests, but specific diagnosis of beta-thalassemia trait often relies on **hemoglobin electrophoresis** or **high-performance liquid chromatography (HPLC)** to quantify the different types of hemoglobin. However, a more readily available and commonly used screening test for beta-thalassemia trait is the **Mentzer index**, calculated as the **Mean Corpuscular Volume (MCV) divided by the Red Cell Distribution Width (RDW)**. An **MCV/RDW ratio < 13** is suggestive of beta-thalassemia trait. ## Why Each Wrong Option is Incorrect - **Option A:** This option is incorrect because, although CBC provides information on MCV, RDW, and other parameters, it does not specifically diagnose beta-thalassemia trait on its own without further calculations or testing. - **Option B:** This option might seem plausible but is not specific or sensitive enough for the diagnosis of beta-thalassemia trait compared to other methods. - **Option D:** Without specifics on what "D" entails, it's hard to directly refute, but typically, more specific tests like hemoglobin electrophoresis are definitive. ## Clinical Pearl / High-Yield Fact A key clinical pearl is that a **Mentzer index < 13** can help differentiate beta-thalassemia trait from iron deficiency anemia, with the former typically having a lower MCV/RDW ratio. This is crucial because both conditions present with microcytic anemia but require different management strategies. ## Correct Answer: C. Mentzer index.