Which of the following clinical manifestations is often observed in individuals with severe combined immunodeficiency (SCID)?

Explanation: The correct answer is b. Genetic or inherited defects. Immunodeficiency disorders are primarily caused by genetic mutations or defects that result in a weakened or impaired immune system.

Incorrect choices:

a. While environmental factors can contribute to the development of secondary immunodeficiency disorders, they are not the primary cause of most immunodeficiency disorders.

c. Autoimmune reactions are not the primary cause of immunodeficiency disorders. Autoimmune disorders involve the immune system attacking the body's own tissues, not a deficiency in the immune system.

d. Excessive production of white blood cells is not the primary cause of immunodeficiency disorders. In fact, immunodeficiency disorders are characterized by a deficiency or impairment of white blood cells, which are essential components of the immune system.

The correct answer is c. Ataxia-telangiectasia. Ataxia-telangiectasia is a rare genetic disorder that affects the cerebellum, causing poor muscle coordination (ataxia) and small, dilated blood vessels (telangiectasias). It also leads to defective T-cell function, making affected individuals more susceptible to infections.

Incorrect choices:

a. X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disorder that primarily affects B-cell function, leading to a lack of mature B cells and low levels of immunoglobulins, but does not involve defective T-cell function.

b. Selective immunoglobulin A (IgA) deficiency is characterized by a deficiency of IgA, but it does not typically lead to defective T-cell function.

d. DiGeorge syndrome is caused by a deletion on chromosome 22 and primarily affects the development of certain organs, including the thymus and parathyroid glands, resulting in T-cell deficiency. While it may lead to some B-cell abnormalities, defective T-cell function is the hallmark of this disorder.