An 80-year-old man presents with fatigue and isolated lymphocytosis. What is the most likely diagnosis?

The presentation of an 80-year-old man with fatigue and isolated lymphocytosis strongly suggests a diagnosis of chronic lymphocytic leukemia (CLL). CLL is a common hematologic malignancy that primarily affects older adults, typically presenting with lymphocytosis, which is characterized by an elevated lymphocyte count in the peripheral blood.

In this case, the isolated lymphocytosis is particularly indicative because CLL is known for causing a proliferation of small, mature lymphocytes. Fatigue is a common symptom due to the disease's effects on the bone marrow and the body's overall blood cell production.

Other conditions mentioned have different characteristic features. For instance, multiple myeloma typically presents with symptoms related to bone pain, renal insufficiency, anemia, and increased levels of proteins in the blood rather than isolated lymphocytosis. Acute myeloid leukemia (AML) would often manifest with a variety of cytopenias and potentially an elevated white blood cell count, but not typically isolated lymphocytosis. Polycythemia vera is associated with elevated red blood cell mass and often presents with symptoms related to hyperviscosity, rather than just lymphocyte elevation.

Thus, the combination of age, fatigue, and isolated lymphocytosis aligns well with chronic