A 69-year-old woman is brought to the emergency room after falling off a step stool and fracturing her hip. Her past medical history is significant for several bouts of pneumonia during the past year. Laboratory results indicate a normal white blood cell count, but platelets are decreased, and the erythrocyte sedimentation rate (ESR) is elevated. X-rays reveal multiple lytic bone lesions. Serum electrophoresis demonstrates an M-protein spike. Which of the following is the MOST likely diagnosis?

**Core Concept**
The patient's presentation suggests a plasma cell dyscrasia, characterized by the proliferation of malignant plasma cells that produce monoclonal immunoglobulins (M-proteins). The lytic bone lesions, anemia (suggested by the elevated ESR), and decreased platelets are indicative of a systemic disease process.

**Why the Correct Answer is Right**
The combination of lytic bone lesions, anemia, and decreased platelets is highly suggestive of multiple myeloma, a type of plasma cell dyscrasia. The elevated ESR and decreased platelets are indicative of anemia-related complications. The presence of an M-protein spike on serum electrophoresis further supports this diagnosis. Multiple myeloma is characterized by the proliferation of malignant plasma cells in the bone marrow, leading to the production of excessive amounts of monoclonal immunoglobulins, which can cause lytic bone lesions, anemia, and thrombocytopenia.

**Why Each Wrong Option is Incorrect**
**Option A:** While chronic lymphocytic leukemia (CLL) can present with anemia and thrombocytopenia, it is less commonly associated with lytic bone lesions and an M-protein spike on serum electrophoresis.
**Option B:** Waldenström's macroglobulinemia is a type of lymphoplasmacytic lymphoma characterized by the production of IgM monoclonal proteins. While it can present with anemia and thrombocytopenia, it is less commonly associated with lytic bone lesions.
**Option C:** Chronic myeloid leukemia (CML) is characterized by the proliferation of granulocytic cells in the bone marrow. While it can present with anemia and thrombocytopenia, it is less commonly associated with lytic bone lesions and an M-protein spike on serum electrophoresis.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Multiple myeloma is a malignancy of plasma cells in the bone marrow, and its diagnosis is often suggested by the triad of lytic bone lesions, anemia, and hypercalcemia (not mentioned in this case, but a common presentation). Remember the "CRAB" criteria for multiple myeloma: **C**alcium elevation, **R**enal insufficiency, **A**nemia, and **B**one lesions.

**Correct Answer:** C. Multiple myeloma.

✓ Correct Answer: C. Multiple myeloma (plasma cell myeloma)