About magnesium, which of the following is incorrect?
A. Plasma contains only 0.3% of total body magnesium, almost 90% is in bone, muscle and soft tissue. In patients with magnesium deficiency, serum magnesium levels can be normal in face of total body magnesium depletion.
B. Only 55% of the magnesium in plasma is in the ionized (active) form. When the serum magnesium is abnormally low, it is impossible to determine whether the problem is a decrease in the ionized (active) fraction or a decrease in the bound fractions (e.g., hypoproteinemia).
C. The hypocalcemia that accompanies magnesium depletion is due to impaired parathormone release combined with an impaired end-organ response to parathormone. Magnesium deficiency may also act on bone directly to reduce calcium release.
D. If hypophosphatemia is ever found, it is unlikely to be related to magnesium metabolism.
Answer: Hypophosphatemia is a cause rather than effect of magnesium depletion. The mechanism is enhanced renal magnesium excretion. Therefore, when hypophosphatemia accompanies hypomagnesemia, the phosphate stores should be replenished to ensure adequate repletion of magnesium stores.
Reference
Paul L. Marino. THE ICU BOOK. Second Edition