Final answer:
The symptoms presented suggest lymphoma, with a biopsy being the best initial test for diagnosis. Treatment typically involves chemotherapy and potentially immunotherapy, yielding a good prognosis with early detection.
Step-by-step explanation:
The clinical presentation described in the question suggests a diagnosis of lymphoma, specifically Diffuse Large B-Cell Lymphoma (DLBCL). The painless, rubbery lymph nodes, drenching fevers, and significant weight loss are all hallmark symptoms. The best initial test would likely be a biopsy of one of the enlarged lymph nodes to confirm the diagnosis and to determine the specific type of lymphoma. Treatment for lymphoma usually involves chemotherapy, and frequently, immunotherapy may be added. The prognosis can be good with early detection and appropriate treatment, as about 70% of DLBCL patients can potentially be cured.