**Core Concept**
Beta thalassemia is a genetic disorder characterized by mutations in the HBB gene that encodes the beta-globin subunit of hemoglobin. This mutation leads to reduced or absent production of the beta-globin chain, resulting in hemoglobinopathies and anemia.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of beta thalassemia. In this condition, the beta-globin chain is either reduced or absent, leading to an imbalance in the production of alpha and beta-globin chains. This imbalance results in the formation of abnormal hemoglobin molecules, such as hemoglobin H and hemoglobin Bart's. The absence of the beta-globin chain also leads to the accumulation of alpha-globin chains, which can form insoluble aggregates and contribute to the anemia and other clinical manifestations of beta thalassemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary defect in beta thalassemia.
**Option B:** This option is incorrect because it is more characteristic of alpha thalassemia, which is a different genetic disorder.
**Option C:** This option is incorrect because it is a feature of other genetic disorders, such as sickle cell anemia, but not beta thalassemia.

**Clinical Pearl / High-Yield Fact**
Beta thalassemia is often inherited in an autosomal recessive pattern, and individuals with two copies of the mutated gene (one from each parent) are more likely to develop the severe form of the disease. Understanding the genetic basis of beta thalassemia is crucial for diagnosis and management.

**Correct Answer:** D.