**Core Concept**
Bart hemoglobin is a variant of hemoglobin that is responsible for alpha-thalassemia major, a severe form of thalassemia characterized by the absence or significant reduction of alpha-globin chains. This variant has implications for the structure and function of the hemoglobin molecule.

**Why the Correct Answer is Right**
Bart hemoglobin is a tetramer composed of four polypeptide chains, specifically four alpha-globin chains. This composition is distinct from normal adult hemoglobin, which consists of two alpha-globin chains and two beta-globin chains. The absence of beta-globin chains in Bart hemoglobin results in a loss of function, leading to severe anemia and other complications. The alpha-globin chains in Bart hemoglobin are unable to effectively bind oxygen, making it difficult for the body to transport oxygen to tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Bart hemoglobin is composed of alpha-globin chains, not beta-globin chains.
**Option B:** Incorrect because Bart hemoglobin is a tetramer, not a dimer.
**Option C:** Incorrect because Bart hemoglobin is composed of alpha-globin chains, not gamma-globin chains.

**Clinical Pearl / High-Yield Fact**
Bart hemoglobin is an example of a pathogenic variant that results in a severe form of thalassemia. It is essential to recognize this variant and its implications for patient care, including the need for frequent blood transfusions and other supportive therapies.

**Correct Answer:** A. A tetramer of four alpha-globin chains.