**Core Concept**
Hairy cell leukemia is a rare, chronic, and malignant disorder of the blood characterized by an accumulation of abnormal B lymphocytes in the bone marrow, spleen, and liver. These neoplastic cells exhibit distinctive "hairy" projections on their surface, which are actually long, thin extensions of the cell membrane.

**Why the Correct Answer is Right**
Hairy cell leukemia is caused by an abnormality in the _BCR-ABL1_ gene, although the exact pathogenesis is still unclear. The disease is characterized by a triad of symptoms: splenomegaly, anemia, and neutropenia. The neoplastic cells in hairy cell leukemia are typically positive for tartrate-resistant acid phosphatase (TRAP) and CD103, which helps in their identification.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hairy cell leukemia is a type of lymphoid leukemia, not a myeloproliferative disorder.
**Option B:** This option is incorrect because hairy cell leukemia is not characterized by the presence of Reed-Sternberg cells, which are typically seen in Hodgkin lymphoma.
**Option C:** This option is incorrect because hairy cell leukemia is not a type of plasma cell disorder, such as multiple myeloma or Waldenström macroglobulinemia.

**Clinical Pearl / High-Yield Fact**
Hairy cell leukemia is often associated with a good prognosis and can be effectively treated with chemotherapy, immunotherapy, or splenectomy. A characteristic feature of this disease is the presence of "hairy" cells in the bone marrow and spleen, which can be identified on histopathological examination.

**Correct Answer: B. Hairy cell leukemia is a type of lymphoid leukemia characterized by an accumulation of abnormal B lymphocytes in the bone marrow, spleen, and liver.**