A 60-year-old man is referred because of splenomegaly and generalized lymphadenopathy. The total white blood cell count is markedly elevated, and the differential count reveals a preponderance of mature appearing lymphocytes. Bone marrow examination reveals a diffuse infiltration with similar-appearing lymphocytes. Which of the following statements best characterizes this disorder?

**Core Concept**
Hairy cell leukemia is a rare, chronic malignancy characterized by an abnormal proliferation of mature-appearing lymphocytes in the bone marrow, spleen, and lymph nodes. It is a distinct entity within the spectrum of lymphoproliferative disorders.

**Why the Correct Answer is Right**
The hallmark feature of hairy cell leukemia is the presence of hairy cells, which are mature B lymphocytes with characteristic hair-like projections on their surface. These cells accumulate in the bone marrow, spleen, and lymph nodes, leading to splenomegaly, lymphadenopathy, and leukocytosis. The hairy cells are often seen in clusters and have a distinctive appearance under the microscope. The disease is typically associated with a poor prognosis and a variable response to treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** 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. While both conditions can present with lymphadenopathy and splenomegaly, the histological features and clinical course are distinct.
**Option B:** This option is incorrect because chronic lymphocytic leukemia (CLL) is a different entity characterized by a predominance of mature lymphocytes in the peripheral blood and bone marrow, but it typically does not present with hairy cells and has a more indolent course.
**Option C:** This option is incorrect because follicular lymphoma is a distinct type of non-Hodgkin lymphoma characterized by a specific histological appearance and a different clinical behavior.
**Option D:** This option is incorrect because acute lymphoblastic leukemia (ALL) is a more aggressive malignancy characterized by the presence of immature lymphoblasts in the bone marrow and peripheral blood, which is not consistent with the findings in hairy cell leukemia.

**Clinical Pearl / High-Yield Fact**
Hairy cell leukemia is often associated with pancytopenia due to bone marrow infiltration, and patients may present with symptoms such as fatigue, weight loss, and recurrent infections.

**Correct Answer:** C.

✓ Correct Answer: B. Bacterial infections are common early in the disease due to hypogammaglobulinemia