**Core Concept**
Memory T cells are a subset of T lymphocytes that have previously encountered an antigen and are able to mount a rapid and effective immune response upon subsequent exposure. They play a crucial role in long-term immunity and are characterized by their ability to proliferate and differentiate into effector cells more quickly than naive T cells.

**Why the Correct Answer is Right**
Memory T cells can be identified by the expression of the CD45RO marker, which is a distinct isoform of the CD45 protein that is expressed on the surface of memory T cells. This marker is a critical component of the T cell receptor complex and is involved in the signaling events that occur upon T cell activation. The CD45RO marker is useful for distinguishing memory T cells from naive T cells, which express a different isoform of CD45.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as CD4 is a marker expressed on T helper cells, but not all memory T cells are T helper cells.
**Option B:** Incorrect, as CD8 is a marker expressed on cytotoxic T cells, but not all memory T cells are cytotoxic T cells.
**Option C:** Incorrect, as CD19 is a marker expressed on B cells, not T cells.

**Clinical Pearl / High-Yield Fact**
Memory T cells can be further divided into two subsets: central memory T cells (Tcm) and effector memory T cells (Tem). Tcm cells are primarily involved in the maintenance of immune homeostasis and are found in the lymphoid tissues, while Tem cells are more involved in the rapid response to pathogens and are found in the peripheral tissues.

**Correct Answer: C. CD19 is incorrect. CD19 is a marker for B cells, not T cells, so it is not the correct answer.