A 2-year-old with moderate splenomegaly, lymphocytes with cytoplasmic projections, and tartrate-resistant acid phosphatase positivity. Diagnosis?

• A. Chronic lymphocytic leukemia
• B. Acute lymphoblastic leukemia
• C. Hairy cell leukemia
• D. Chronic myeloid leukemia

Answer: (C)

The key idea is recognizing hairy cell leukemia by two classic features: lymphocytes with cytoplasmic projections that give a “hairy” appearance and tartrate-resistant acid phosphatase (TRAP) positivity. Hairy cell leukemia is a mature B-cell neoplasm where the cells extend fine cytoplasmic processes, and these cells stain positive for TRAP. This combination is highly characteristic and points to hairy cell leukemia, even though it’s more common in adults; pediatric cases can occur and present with splenomegaly.

In this context, the splenomegaly fits with hairy cell leukemia due to red pulp infiltration. TRAP positivity further supports the diagnosis, whereas the other leukemias typically do not show this staining pattern. Acute lymphoblastic leukemia would present with blasts rather than mature hairy cells, and chronic myeloid leukemia involves myeloid proliferation with different markers and does not show TRAP positivity or the hairy cell morphology. Chronic lymphocytic leukemia likewise involves small mature-appearing lymphocytes in older individuals and is not associated with TRAP positivity.