## Core Concept
Sickle cell anemia is a genetic disorder characterized by the production of abnormal hemoglobin, known as hemoglobin S (HbS), which leads to the deformation of red blood cells under certain conditions. This condition results from a point mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin, causing the substitution of glutamic acid with valine at position 6. The abnormal hemoglobin polymerization under low oxygen tension causes red blood cells to assume a sickle shape.

## Why the Correct Answer is Right
The correct answer, , relates to the understanding that sickle cell anemia is caused by a specific mutation leading to the production of abnormal hemoglobin. This results in sickling of red blood cells, which can cause a variety of clinical issues including vaso-occlusive crises, hemolytic anemia, and increased susceptibility to infections.

## Why Each Wrong Option is Incorrect
- **Option A:** While specific details about option A are not provided, any statement that aligns with established facts about sickle cell anemia would be correct, and thus not the answer we're looking for here.
- **Option B:** Similarly, without specifics on option B, we acknowledge that accurate descriptions of sickle cell anemia's pathophysiology, clinical manifestations, or genetic basis would not be the "except" we're seeking.
- **Option C:** This option would contain incorrect information regarding sickle cell anemia. For example, if it stated that sickle cell anemia is caused by an acquired mutation or that it's not associated with hemoglobin S, it would be incorrect.
- **Option D:** Like option C, without details, any statement contradicting known facts about sickle cell anemia (e.g., its autosomal recessive inheritance pattern, its prevalence in certain populations) would make it an incorrect statement.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sickle cell anemia is an autosomal recessive disorder. This means that a person must inherit two abnormal copies of the HBB gene (one from each parent) to express the disease. Individuals with one normal and one abnormal copy are considered carriers, known as sickle cell trait, and typically have a milder phenotype but can pass the abnormal gene to their offspring.

## Correct Answer: D.