99. Which one of the immunodeficiency is caused by a mutation in BTK gene:
A. X-linked SCID
B. X-linked agammaglobulinemia
C. DiGeorge syndrome
D. Chronic granulomatous disease

Detailed Explanation:

Immunodeficiencies are conditions in which the immune system’s ability to fight infections is impaired. One such condition is X-linked agammaglobulinemia (XLA), a rare inherited disorder that affects the immune system’s ability to produce antibodies. It is caused by a mutation in the Bruton tyrosine kinase (BTK) gene, which is located on the X chromosome. This mutation leads to a deficiency in B cells, which are responsible for producing antibodies in response to infections.

Understanding X-linked Agammaglobulinemia (XLA):

XLA is characterized by the absence or severe reduction of circulating B cells, which in turn leads to a lack of immunoglobulins (antibodies) in the blood. The BTK gene plays a crucial role in B cell development and differentiation. Mutations in the BTK gene prevent B cells from maturing properly, resulting in the absence of functional B cells and antibodies.

Symptoms of XLA often appear in early childhood and include frequent bacterial infections, especially in the respiratory and gastrointestinal systems. Without treatment, children with XLA are at a higher risk of severe infections, including pneumonia and sepsis.

Diagnosis is confirmed through genetic testing, which identifies mutations in the BTK gene, and laboratory tests that show low levels of immunoglobulins in the blood and a lack of mature B cells.

Treatment typically involves immunoglobulin replacement therapy, which provides the antibodies the patient cannot produce on their own. This helps prevent infections and improves the quality of life. Antibiotics may also be used to treat infections promptly.

Key Point:

X-linked agammaglobulinemia is caused by a mutation in the BTK gene, leading to defective B cell maturation and antibody production.

Final Answer:

B. X-linked agammaglobulinemia