CD 19 positive, CD 22 positive, CD103 positive monoclonal B-cells with bright kappa positivity were found to comprise 60% of the peripheral blood lymphoid cells on flow cytometric analysis in a 55 year old man with massive splenomegaly and a total leucocyte count of 3.3 × 109/L. Which one of the following is the most likely diagnosis?

**Core Concept**
A monoclonal B-cell proliferation with specific surface marker expression is suggestive of a B-cell lymphoproliferative disorder. CD19 and CD22 are pan-B-cell markers, while CD103 is often expressed in hairy cell leukemia. Bright kappa positivity indicates monoclonal B-cell expansion.

**Why the Correct Answer is Right**
The patient's presentation of massive splenomegaly and a high percentage of monoclonal B-cells on flow cytometry is consistent with hairy cell leukemia. This neoplastic disorder is characterized by the accumulation of CD103-positive, CD19-positive, and CD22-positive B-cells in the spleen and bone marrow. The bright kappa positivity reflects the clonal nature of the B-cell expansion. Hairy cell leukemia typically presents with a low total leukocyte count, which is consistent with the patient's count of 3.3 × 10^9/L.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chronic Lymphocytic Leukemia (CLL) typically presents with CD5 and CD23 positivity, which are not mentioned in the patient's flow cytometry results.
* **Option B:** Follicular lymphoma is characterized by a follicular pattern on histology and typically expresses CD10, BCL-6, and MUM-1, which are not mentioned in the patient's flow cytometry results.
* **Option D:** Mantle cell lymphoma typically expresses CD5 and CD20, but not CD103. The patient's flow cytometry results do not support this diagnosis.

**Clinical Pearl / High-Yield Fact**
Hairy cell leukemia is often associated with a low total leukocyte count, despite significant splenomegaly, due to the neoplastic cells' accumulation in the spleen and bone marrow.

**Correct Answer:** C. Hairy cell leukemia.

✓ Correct Answer: D. Hairy cell leukemia