**Question:** Most severe form of hereditary spherocytosis is caused by the mutation of:

A. SLC4A1
B. HBB
C. HFE
D. POFUT1

**Core Concept:**
Hereditary spherocytosis (HS) is a group of inherited red blood cell disorders characterized by the formation of spherical, or spherocytes, due to abnormal band 3 anion transporter (band 3 is encoded by the SLC4A1 gene). Spherocytes are fewer in number and more fragile than normal red blood cells. This leads to hemolysis, anemia, and jaundice. In HS, patients may present with compensatory splenomegaly, elevated serum bilirubin levels, and microcytic anemia.

**Why the Correct Answer is Right:**
The correct answer, A. SLC4A1, refers to the gene that encodes the anion exchanger 1 protein, also known as band 3. This protein plays a crucial role in maintaining the membrane integrity of red blood cells (RBCs). Mutations in SLC4A1 can lead to HS, making it the correct choice for the most severe form of the disease.

**Why Each Wrong Option is Incorrect:**

B. HBB (Hemoglobin B) refers to the gene that encodes beta-globin subunit of hemoglobin. Mutations in this gene cause hemoglobinopathies like beta-thalassemia, not HS.

C. HFE (Hemochromatosis E) encodes the hemojuvelin protein involved in iron absorption. Mutations in HFE are associated with hereditary hemochromatosis, not HS.

D. POFUT1 (fucosylphosphatidylinositol-anchored protein O-fucosyltransferase 1) is a gene responsible for adding fucose to proteins embedded in the cell membrane. Mutations in POFUT1 cause a rare disorder called hereditary elliptocytosis, not HS.

**Clinical Pearl:**
Hereditary spherocytosis is a genetically determined disorder affecting the red blood cell membrane proteins, particularly the band 3 anion exchanger protein encoded by the SLC4A1 gene. The disease is characterized by hemolysis, anemia, and splenomegaly. Early diagnosis and treatment can prevent complications like portal hypertension and liver fibrosis.