This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: miscellaneous, Subject name: Biochemistry
Anion gap means 
Select and explain. 
A. Gap in anion molecule, B. Difference between high and normal anion gap acidosis, C. Unmeasured ions, D. Primary deficit of anions
C: Unmeasured ions
Answer: C: Unmeasured ions 
Explanation: i. The sum of cations and anions in ECF is always equal, so as to maintain the electrical neutrality. Sodium and potassium together account for 95% of the cations whereas chloride and bicarbonate account for only 86% of the anions. Only these electrolytes are commonly measured.ii. Hence, there is always a difference between the measured cations and the anions. The unmeasured anions constitute the anion gap. This is due to the presence of protein anions, sulphate, phosphate and organic acids. iii. The anion gap is calculated as the difference between (Na+ + K+) and (HCO3- + Cl-). Normally this is about 12 mmol/L.Ref: D M Vasudevan 7th edition Page no: 399