A 61-year-old man gives a history of back pain for several months. A plain film radiograph of the spine reveals several 1 to 2 cm lytic lesions of the veebral bodies. He has no lymphadenopathy or splenomegaly on physical examination. Serum chemistries show sodium 140 mmol/L, potassium 4.4 mmol/L, chloride 101 mmol/L, CO2 26 mmol/L, glucose 78 mg/dL, urea nitrogen 49 mg/dL, creatinine 5 mg/dL, total protein 8.3 g/dL, albumin 3.7 g/dL, alkaline phosphatase 176 U/L, AST 45 U/L, ALT 22 U/L, and total bilirubin 1.2 mg/dL. He recently developed a cough productive of yellowish sputum, and a sputum culture grew Streptococcus pneumoniae. During the course of his hospitalization, a bone marrow biopsy is performed. The biopsy is most likely to show:

**Core Concept**
The patient's presentation of lytic bone lesions, anemia (suggested by low albumin and elevated urea nitrogen), and renal impairment (elevated creatinine) in the context of a productive cough and a positive sputum culture for Streptococcus pneumoniae is suggestive of a myeloproliferative neoplasm. Specifically, the combination of bone marrow failure, anemia, and renal impairment, along with the presence of lytic bone lesions, is characteristic of multiple myeloma.

**Why the Correct Answer is Right**
Multiple myeloma is a malignancy of plasma cells that accumulate in the bone marrow, leading to bone destruction and lytic lesions. The patient's elevated alkaline phosphatase, AST, and ALT suggest liver involvement, possibly due to hepatic metastases or a paraneoplastic syndrome. The presence of Streptococcus pneumoniae in the sputum culture is consistent with immunocompromised state due to multiple myeloma. The bone marrow biopsy is expected to show a proliferation of malignant plasma cells, which can be confirmed by immunohistochemical staining.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the expected findings in multiple myeloma. A bone marrow biopsy in multiple myeloma would typically show a clonal proliferation of plasma cells, not a decrease in plasma cells.

**Option B:** This option is incorrect as it does not accurately describe the expected findings in multiple myeloma. A bone marrow biopsy in multiple myeloma would typically show a clonal proliferation of plasma cells, not an increase in lymphocytes.

**Option D:** This option is incorrect as it does not accurately describe the expected findings in multiple myeloma. A bone marrow biopsy in multiple myeloma would typically show a clonal proliferation of plasma cells, not an increase in granulocytes.

**Clinical Pearl / High-Yield Fact**
The presence of lytic bone lesions and renal impairment in the context of a productive cough and a positive sputum culture for Streptococcus pneumoniae should raise suspicion for multiple myeloma. This condition is often diagnosed incidentally on a routine radiograph or during evaluation for anemia or renal impairment.

**Correct Answer:** C. Clonal proliferation of malignant plasma cells.

✓ Correct Answer: D. Numerous plasma cells