Which disease is caused by a mutation in SAP?

The disease caused by a mutation in the SAP (signaling lymphocytic activation molecule-associated protein) gene is X-linked lymphoproliferative syndrome. This condition is characterized by an abnormal immune response to Epstein-Barr virus (EBV), leading to a range of clinical manifestations, including lymphoproliferation, severe immunodeficiency, and increased susceptibility to certain malignancies.

Patients with this syndrome typically present with symptoms related to EBV infection, which can manifest as infectious mononucleosis, lymphomas, and various hematological conditions. The critical role of SAP in the immune system highlights why a mutation in this gene impacts the body's ability to manage viral infections effectively.

In contrast, the other options listed correspond to different underlying genetic causes or mechanisms. For example, X-linked agammaglobulinemia results from mutations in the BTK gene, while severe combined immunodeficiency can arise from mutations in several different genes including ADA, IL2RG, and others. Common variable immunodeficiency involves multiple genetic factors and is characterized by a lack of antibody production but is not linked specifically to SAP mutations.