## Core Concept
Hemoglobin H (HbH) disease is a type of thalassemia, which is a genetic disorder affecting hemoglobin synthesis. It results from a mutation in three out of four globin genes, leading to a significant reduction in the production of the beta-globin chains of hemoglobin. This condition is characterized by the formation of tetramers of beta-globin chains, which are unstable and precipitate within red blood cells.

## Why the Correct Answer is Right
HbH disease is caused by a deletion of three alpha-globin genes or a combination of two alpha-globin gene deletions and a point mutation in the third alpha-globin gene. This results in a severe reduction of alpha-globin chain production. Without sufficient alpha-globin chains, beta-globin chains accumulate and form tetramers (β4), which are called hemoglobin H. This condition leads to microcytic, hypochromic anemia.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe the characteristics of HbH disease.
- **Option B:** This option is incorrect as it does not specify the correct globin chain composition or the nature of HbH disease.
- **Option C:** Although not directly provided, any option suggesting a different globin chain imbalance or a different type of hemoglobinopathy would be incorrect because HbH is specifically characterized by the formation of β4 tetramers due to alpha-globin chain deficiency.

## Clinical Pearl / High-Yield Fact
A key clinical feature of HbH disease is the presence of **inclusion bodies** within red blood cells due to the precipitation of beta-globin tetramers. These inclusions are often seen on a reticulocyte count or after splenectomy. Clinically, patients with HbH disease often present with moderate to severe anemia, jaundice, and splenomegaly.

## Correct Answer: D. β4 tetramer.