A 73-year-old patient presents to a primary care physician with complaints of fatigue, unexplained weight loss, and abdominal discomfort for the past two months. On physical examination, the patient has an enlarged spleen, and laboratory tests show pancytopenia with a white blood cell count of 2,000/ µL, hemoglobin of 9.5 g/dL, and platelet count of 50,000/µL. A bone marrow biopsy is performed and reveals the presence of "hairy" lymphocytes. What disease is the likely cause of this patient's symptoms?