What laboratory feature distinguishes hairy cell leukemia?

Hairy cell leukemia is characterized by the presence of atypical lymphocytes with "hairy" cytoplasmic projections when observed under a microscope. One key laboratory feature that helps distinguish hairy cell leukemia is the positive staining with acid phosphatase, particularly the tartrate-resistant form. This enzyme activity is often elevated in malignant cells, and its detection is important in the diagnosis of hairy cell leukemia.

The other choices do not specifically correlate with hairy cell leukemia. A high white blood cell count can be seen in various types of leukemias and is not unique to hairy cell leukemia. The Philadelphia chromosome is primarily associated with chronic myeloid leukemia and not hairy cell leukemia. Excessive eosinophils are typically linked to other conditions, such as allergic reactions or certain infections, not specifically to hairy cell leukemia. Thus, the positive staining with acid phosphatase is indeed the distinguishing laboratory feature for this condition.