**Core Concept**
The selective decrease in IgA refers to a reduction in the levels of immunoglobulin A, a type of antibody, without affecting other immunoglobulin classes. This phenomenon can be observed in certain clinical conditions or as a result of specific treatments.

**Why the Correct Answer is Right**
The correct answer involves a drug that specifically targets the production or function of IgA. The mechanism behind this effect is related to the inhibition of IgA production or its secretion. One of the key molecules involved in the regulation of IgA production is the enzyme TACI (Transmembrane Activator and CAML Interactor), which is a receptor for the cytokines BAFF (B cell-activating factor) and APRIL (A Proliferation-Inducing Ligand). These cytokines play a crucial role in the differentiation and survival of plasma cells that produce IgA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically target IgA production or function. The effects of this drug are more general and can affect multiple types of antibodies.

**Option B:** This option is incorrect because it has a different mechanism of action that is not related to the selective decrease in IgA. It involves the inhibition of a different enzyme that is involved in the production of other types of antibodies.

**Option C:** This option is incorrect because it does not specifically target IgA production or function. The effects of this drug are more general and can affect multiple types of antibodies.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the selective decrease in IgA can be observed in various clinical conditions, including IgA nephropathy, which is a disease characterized by the deposition of IgA antibodies in the kidneys. This condition can lead to kidney damage and failure if left untreated.

**Correct Answer: C. Rituximab**