**Question:** The hemoglobin types found in a normal adult are:

A. Hemoglobin A
B. Hemoglobin A2
C. Hemoglobin F
D. Hemoglobin S

**Correct Answer:**

**Core Concept:** Hemoglobin is the oxygen-carrying protein found in red blood cells (RBCs), which transports oxygen from the lungs to the body's tissues and returns carbon dioxide back to the lungs. The main types of adult human hemoglobin are hemoglobin A, A2, F, and S.

**Why the Correct Answer is Right:**

Hemoglobin A (HbA) is the most common form in adults, comprising approximately 95-98% of adult hemoglobin. It consists of two alpha (α) and two beta (β) globin chains. Hemoglobin A2 (HbA2) makes up about 1-3%, and it consists of two delta (δ) and two beta (β) globin chains. Hemoglobin F (HbF) is found in lower levels, making up approximately 0.5-2%, and it consists of two gamma (γ) and two beta (β) globin chains. Hemoglobin S (HbS) is the major component of sickle cell anemia, a genetic disorder.

**Why Each Wrong Option is Incorrect:**

A. Hemoglobin D (HbD) is a rare variant found in some populations, making up <1%, and consists of two alpha (α) and two delta (δ) globin chains. This is not one of the main adult hemoglobin types. B. Hemoglobin C (HbC) is another rare variant, making up <1% of hemoglobin, and consists of two alpha (α) and two gamma (γ) globin chains. It is not considered a primary adult hemoglobin type. C. Hemoglobin E (HbE) is a rare variant found in Southeast Asian populations, making up <1% of hemoglobin, and consists of two alpha (α) and two epsilon (ε) globin chains. It is also not a primary adult hemoglobin type. D. Hemoglobin O (HbO), also known as hemoglobin F, is not a distinct hemoglobin type but refers to the presence of hemoglobin F in an adult. This option is incorrect because it does not represent a separate hemoglobin type. **Clinical Pearl:** A better way to phrase this would be to mention the clinical significance of the presence of HbF in adults, which is associated with favorable outcomes in sickle cell anemia and beta-thalassemia. In these conditions, adult HbF helps maintain adequate oxygen-carrying capacity despite reduced alpha-globin chain synthesis.