**Core Concept**
Memory T cells are a subset of T lymphocytes that have been previously exposed to antigens and can mount a rapid and effective immune response upon re-exposure. They are characterized by their ability to differentiate and proliferate more quickly than naive T cells.

**Why the Correct Answer is Right**
CD45RO is a specific isoform of the CD45 protein, a transmembrane glycoprotein that serves as a receptor-linked protein tyrosine phosphatase. CD45RO is expressed on the surface of memory T cells, including both central memory T cells (TCM) and effector memory T cells (TEM), and plays a crucial role in their activation and proliferation. The expression of CD45RO is a key marker for identifying memory T cells, distinguishing them from naive T cells, which express CD45RA.

**Why Each Wrong Option is Incorrect**
**Option A:** CD45RA is expressed on naive T cells, not memory T cells, and is used as a marker to identify cells that have not yet encountered antigens.
**Option B:** CD45RB is involved in the regulation of T cell activation and proliferation, but it is not a specific marker for memory T cells.
**Option C:** CD45RC is not a commonly used marker for identifying memory T cells, and its expression is not as well-characterized as CD45RO.

**Clinical Pearl / High-Yield Fact**
Memory T cells play a critical role in long-term immunity and are essential for controlling chronic infections and preventing disease recurrence. Understanding the markers and characteristics of memory T cells is crucial for developing effective immunotherapies and vaccines.

**Correct Answer:**
✓ Correct Answer: D. CD45RO