**Core Concept**
Felty's syndrome is a rare but serious complication of long-standing rheumatoid arthritis (RA), characterized by the classic triad of rheumatoid arthritis, splenomegaly, and anemia due to hypersplenism. This condition is often associated with an increased risk of infections and hematological abnormalities.

**Why the Correct Answer is Right**
Felty's syndrome is a complex condition involving the interplay of various immune and hematological mechanisms. The classic triad includes rheumatoid arthritis, splenomegaly, and anemia. Rheumatoid arthritis is an autoimmune disease that leads to inflammation and joint damage. Splenomegaly, or enlargement of the spleen, is thought to be caused by the sequestration of red blood cells and other hematopoietic cells in the spleen, leading to anemia. The exact mechanisms underlying Felty's syndrome are not fully understood, but it is believed to involve the abnormal activation of T lymphocytes and the production of autoantibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** Rheumatoid arthritis is a manifestation of Felty's syndrome, as it is one of the classic components of the triad.
**Option B:** Splenomegaly is a manifestation of Felty's syndrome, resulting from the sequestration of hematopoietic cells in the spleen.
**Option C:** Anemia is a manifestation of Felty's syndrome, often resulting from hypersplenism and the sequestration of red blood cells.

**Clinical Pearl / High-Yield Fact**
Remember that Felty's syndrome is a rare but serious complication of long-standing rheumatoid arthritis, often associated with increased risk of infections and hematological abnormalities. The classic triad of rheumatoid arthritis, splenomegaly, and anemia should be recognized in patients with RA.

**Correct Answer:** D