An elderly patient with cancer presents with back pain, confusion, and constipation. What electrolyte abnormality is most likely?

• A. Hyperkalemia
• B. Hypercalcemia
• C. Hyponatremia
• D. Hypocalcemia

Answer: (B)

Hypercalcemia due to malignancy best explains this presentation. In cancer, especially with bone involvement, tumors can cause osteolytic bone resorption and release calcium into the blood, and many tumors secrete PTH-related peptide which raises calcium levels. The result is symptoms like constipation and confusion, along with bone pain from metastases or bone turnover. The back pain points to bone involvement, while the GI and CNS effects align with elevated calcium. Other electrolyte problems don’t fit as well: hyperkalemia would cause weakness and cardiac issues; hyponatremia mainly causes confusion without the prominent constipation; hypocalcemia produces tingling, cramps, and tetany rather than the constellation seen here.