Which distribution accurately describes magnesium in plasma?

• A. Ionized 70%, Protein bound 20%, Ion bound 10%
• B. Ionized 55%, Protein bound 30%, Ion bound 15%
• C. Ionized 40%, Protein bound 40%, Ion bound 20%
• D. Ionized 60%, Protein bound 25%, Ion bound 15%

Answer: (B)

Magnesium in plasma exists in three forms: the free ionized form, protein-bound magnesium (mostly to albumin), and magnesium that is complexed with anions. The active, physiologically important fraction is the ionized Mg2+, which is typically about 55–60% of the total. The protein-bound portion is around 30%, and the remaining ~10–15% is complexed with anions.

The option that reflects roughly 55% ionized, 30% protein-bound, and 15% ion-bound (complexed) matches these typical proportions, making it the best description. The other distributions either overstate the ionized fraction or understate the protein-bound or complexed portions, which are less consistent with common physiology.