**Core Concept**
Major Histocompatibility Complex (MHC) class II molecules are involved in the presentation of exogenous antigens to CD4+ T cells, playing a crucial role in the initiation of the adaptive immune response. MHC class II molecules are primarily expressed on the surface of antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells.

**Why the Correct Answer is Right**
MHC class II molecules are responsible for the presentation of peptide fragments derived from extracellular pathogens, such as bacteria and viruses, to CD4+ T cells. This process involves the engulfment of pathogens by APCs, the processing of antigens into peptides, and the loading of these peptides onto MHC class II molecules. The MHC class II-peptide complex is then presented to CD4+ T cells, which recognize the antigen through their T-cell receptor (TCR). This recognition event triggers the activation of CD4+ T cells, which subsequently initiate an immune response against the pathogen.

**Why Each Wrong Option is Incorrect**
**Option A:** MHC class II molecules are involved in the presentation of endogenous antigens, which is incorrect. MHC class I molecules are responsible for the presentation of endogenously synthesized antigens to CD8+ T cells.
**Option B:** MHC class II molecules are expressed on the surface of all nucleated cells, which is incorrect. MHC class II molecules are primarily expressed on the surface of APCs such as dendritic cells, macrophages, and B cells.
**Option C:** MHC class II molecules are involved in the presentation of antigens to B cells, which is incorrect. MHC class II molecules are involved in the presentation of antigens to CD4+ T cells, which then activate B cells.

**Clinical Pearl / High-Yield Fact**
MHC class II molecules are essential for the initiation of the adaptive immune response against extracellular pathogens. Understanding the role of MHC class II molecules in antigen presentation is crucial for the development of vaccines and immunotherapies.

**Correct Answer:** Not provided