## Core Concept
The Major Histocompatibility Complex (MHC) is a group of genes that play a critical role in the immune system by encoding proteins responsible for the regulation of the immune response. MHC class I and II molecules are crucial for antigen presentation to T cells. MHC class I molecules are expressed on the surface of almost all nucleated cells, while MHC class II molecules are primarily expressed on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells.

## Why the Correct Answer is Right
MHC class II molecules are primarily involved in the presentation of exogenous antigens to CD4+ T helper cells. They are crucial for initiating the immune response. The correct statement about MHC class II is that they are mainly expressed on APCs and are involved in the presentation of peptide fragments derived from extracellular pathogens to T helper cells.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect statements about MHC class II might include claims that they are expressed on all nucleated cells or that they present endogenous antigens, which is not accurate.
- **Option B:** Similarly, without the specific content, we can infer that if it suggests MHC class II presents endogenous antigens or are expressed on a wide range of cells beyond APCs, it would be incorrect.
- **Option C:** If this option inaccurately describes the function or expression of MHC class II molecules, it would be considered incorrect based on the established roles and characteristics of MHC class II.
- **Option D:** This option being the correct answer implies that the other statements are false regarding MHC class characteristics.

## Clinical Pearl / High-Yield Fact
A key point to remember is that MHC class II molecules are essential for the activation of CD4+ T cells, which then orchestrate the immune response, including the activation of B cells and cytotoxic CD8+ T cells. Defects in MHC class II expression or function lead to severe immunodeficiency, as seen in certain types of bare lymphocyte syndrome.

## Correct Answer: D.