The calcium-phosphorus product threshold above which metastatic calcification can occur is closest to which value?

• A. 30
• B. 40
• C. 55
• D. 70

Answer: (C)

Metastatic calcification happens when the product of serum calcium and phosphate becomes high enough that calcium phosphate begins to precipitate in soft tissues. The threshold commonly cited for this risk is about 55 mg^2/dL^2. So among the values given, 55 is the closest to that threshold. In practice, clinicians try to keep the Ca × P product below roughly 55 to minimize soft-tissue calcification, especially in patients with renal failure or hyperphosphatemia. For context, a calcium of 9 mg/dL and phosphate of 6 mg/dL gives 54, just under the threshold; a product around 60–65 would be increasingly concerning, and around 70 indicates a higher risk.