A patient presents with fatigue, back pain, and recurrent infections; labs show high calcium and a monoclonal spike by electrophoresis. What is the presumptive diagnosis?

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Recognize a plasma cell neoplasm. The combination of fatigue, back pain, recurrent infections, hypercalcemia, and a monoclonal spike on electrophoresis points to a malignant plasma cell disorder that makes a single type of immunoglobulin. In multiple myeloma, malignant plasma cells accumulate in the bone marrow and cause destruction of bone tissue, leading to lytic lesions and calcium release (hypercalcemia), which explains the back pain and high calcium. The monoclonal spike reflects a single clone producing a uniform immunoglobulin, a hallmark feature of this disease. The recurrent infections arise because the abnormal plasma cells crowd out normal antibody production, weakening humoral immunity.

Waldenström macroglobulinemia can also show a monoclonal spike, but it usually presents with hyperviscosity symptoms from an IgM protein and lacks the classic bone-destructive bone lesions and prominent hypercalcemia seen here. Chronic lymphocytic leukemia involves mature lymphocytes and typical findings like lymphadenopathy and cytopenias, not a monoclonal protein spike as the defining feature. Acute myeloid leukemia centers on immature myeloid blasts with cytopenias and organ infiltration, not a monoclonal protein spike.

So the presentation aligns best with multiple myeloma.