## **Core Concept**
The differentiation of human B cells in the bone marrow involves a series of well-orchestrated steps, starting from hematopoietic stem cells. This process includes **V(D)J recombination**, which is crucial for generating a diverse B cell receptor repertoire. The initial steps involve the commitment to the B cell lineage and the rearrangement of immunoglobulin genes.

## **Why the Correct Answer is Right**
The correct answer, **D. Expression of CD19**, marks an early stage in B cell development. CD19 is a marker that appears early in B cell differentiation, even before the B cell receptor is fully assembled. It is one of the first B cell-specific antigens expressed during B cell development in the bone marrow, making it a key marker for early B cell lineage commitment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the events listed do not accurately represent the first step in B cell differentiation.
- **Option B:** This option is incorrect as it does not specify an event that occurs before CD19 expression in B cell development.
- **Option C:** This option is incorrect because it represents a later stage in B cell development.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **CD19** is expressed early in B cell development and is a marker used to identify B cells from the pro-B cell stage through maturity, except in the terminal differentiation to plasma cells.

## **Correct Answer:** D. Expression of CD19.