A patient presents with fatigue, headaches, blurred vision, and recurrent epistaxis; electrophoresis shows a monoclonal spike of IgM. What is the presumptive diagnosis?

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An IgM monoclonal spike with symptoms like fatigue, headaches, blurred vision, and epistaxis points to hyperviscosity syndrome caused by Waldenström macroglobulinemia. In this condition, a lymphoplasmacytic lymphoma produces a large amount of IgM, a pentameric antibody that greatly increases plasma viscosity. The resulting hyperviscosity affects small vessels, especially in the brain and eyes, leading to headaches and blurred vision, and it causes mucosal bleeding such as epistaxis.

This distinguishes it from other plasma cell or lymphoid disorders. Multiple myeloma commonly involves IgG or IgA rather than IgM and is characterized by bone lesions, hypercalcemia, anemia, and renal issues rather than predominantly hyperviscosity. Gammopathy of undetermined significance is asymptomatic with a small M-protein. Chronic lymphocytic leukemia presents with lymphocytosis and characteristic blood smear findings rather than a dominant IgM spike causing hyperviscosity. Waldenström macroglobulinemia is the scenario that best fits both the immunoglobulin profile and the clinical manifestations.