**Core Concept:** Rheumatoid arthritis (RA) is an autoimmune disorder characterized by chronic inflammation of the synovial membrane lining, leading to joint destruction and disability. Infiltration of immune cells is a key feature of RA.

**Why the Correct Answer is Right:**
The correct answer, **D:** Lymphocytes, is a major cell infiltrate in rheumatoid arthritis due to the following reasons:
1. Lymphocytes play a crucial role in immune response and are involved in the pathogenesis of RA.
2. In RA, lymphocytes (specifically T-cells) infiltrate the synovial membrane and synovial fluid, causing synovial inflammation and joint damage.

**Why Each Wrong Option is Incorrect:**
A. **Neutrophils:** Neutrophils are early responders in acute inflammation and are not the primary cell type involved in the chronic inflammation of RA.
B. **Macrophages:** Although macrophages are involved in RA, lymphocytes are more significant in terms of synovial inflammation and joint damage.
C. **Platelets:** Platelets are involved in hemostasis and clot formation, but are not the primary cell type contributing to joint inflammation in RA.

**Clinical Pearl / High-Yield Fact:**
Lymphocytes, particularly T-cells, are crucial in understanding the pathogenesis of rheumatoid arthritis. RA is a complex autoimmune disease involving multiple cell types and inflammatory mediators, which contribute to joint destruction and disability.

**Correct Answer:** **D:** Lymphocytes.