Final answer:
The molecular defect associated with hyper-IgM syndrome is CD154 (CD40 ligand) mutation, which impairs the ability of B cells to switch from IgM to other immunoglobulin isotype production.
Step-by-step explanation:
The molecular defect associated with hyper-IgM syndrome is B. CD154 (CD40 ligand) mutation. This mutation affects the interaction between T cells and B cells, which is crucial for class switching during antibody production. Hyper-IgM syndrome is characterized by normal or elevated levels of IgM antibodies with low levels of other immunoglobulin isotypes like IgG, IgA, and IgE, due to the inability of B cells to switch from IgM production to other types of immunoglobulin production.
The condition detailed in the question, hyper-IgM syndrome, often leads to an increased susceptibility to infections, as patients have a decreased variety of antibody responses. Inherited immunodeficiency diseases such as agammaglobulinemia and severe combined immunodeficiency (SCID) also compromise the immune system, but they are distinct from hyper-IgM syndrome.
A type III hypersensitivity, which involves the accumulation of immune complexes in tissues and small blood vessels, is different from the immunodeficiency disorders discussed but is an example of an immune system malfunction.