## **Core Concept**
Sickle cell disease is a genetic disorder characterized by the production of abnormal hemoglobin, known as **sickle hemoglobin (HbS)**. This abnormal hemoglobin causes red blood cells to assume a sickle shape under certain conditions, leading to a variety of health complications. The disease is inherited in an autosomal recessive pattern.

## **Why the Correct Answer is Right**
The correct answer, , is right because sickle cell disease is indeed caused by a **point mutation in the HBB gene** that codes for the beta-globin subunit of hemoglobin. This mutation results in the substitution of glutamic acid with valine at position 6 of the beta-globin chain, leading to the production of HbS. Under low oxygen conditions, HbS polymerizes, causing red blood cells to become misshapen (sickle-shaped) and rigid, which can lead to vaso-occlusion and hemolysis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while there are various genetic disorders affecting hemoglobin, sickle cell disease is specifically related to a mutation in the HBB gene, not due to a **chromosomal translocation**.
- **Option B:** This option is incorrect as it suggests **multifactorial inheritance**, which does not accurately describe the genetic basis of sickle cell disease. The disease follows an autosomal recessive pattern.
- **Option C:** This option might seem plausible because it mentions **autosomal dominant**, but sickle cell disease is actually inherited in an **autosomal recessive** manner. This means an individual must inherit two copies of the mutated gene (one from each parent) to express the disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that individuals with **sickle cell trait (HbAS)**, who have one normal and one sickle allele, are generally asymptomatic but can pass the sickle cell allele to their offspring. However, they may have a slightly increased risk of certain conditions, such as **urine concentration defects** and **increased susceptibility to certain infections**. It's also crucial to screen for sickle cell disease in populations from areas where the disease is prevalent.

## **Correct Answer:** . C. an autosomal recessive disorder.