Final answer:
The immunodeficiency in Severe Combined Immunodeficiency (SCID) does not stem from a deficiency of stem cells, but from genetic defects that impair both B-cell and T-cell functions. The correct answer is A. A deficiency of stem cells.
Step-by-step explanation:
Severe Combined Immunodeficiency (SCID) is characterized by a deficiency of both T lymphocytes (T cells) and B lymphocytes (B cells), not by a deficiency of stem cells. Therefore, the correct answer is A. A deficiency of stem cells.
SCID is a primary immunodeficiency disease, where patients suffer from defects in both the B-cell and T-cell arms of the adaptive immune response. These deficiencies impair T-cell dependent antibody responses as well as cell-mediated immune responses. Patients with SCID experience severe, often life-threatening, opportunistic infections and do not develop immunological memory, rendering vaccines ineffective. SCID can be caused by various genetic defects, with the X-linked form being most common, primarily affecting males. It is usually diagnosed within the first few months of life.
This condition does not directly result from a deficiency of stem cells, but from genetic mutations that affect the development and function of T and B cells. SCID may involve ineffective stem cell function indirectly, as stem cells give rise to these lymphocytes, but the characterization of SCID does not include stem cell deficiency as a primary feature.