Which of the following is NOT part of dialysate composition?

• A. Purified water
• B. Acid solution
• C. Bicarbonate solution
• D. Urea

Answer: (D)

The idea being tested is what makes up dialysate and why it should carry no waste toxins. Dialysate is designed to create a diffusion gradient that pulls wastes from blood into the dialysate while providing the right electrolytes to keep the patient’s fluid and acid–base balance stable. It is made from purified water plus two concentrates: an acid solution and a bicarbonate solution. The acid concentrate supplies the necessary ions to maintain pH and electrolyte levels, and the bicarbonate solution provides bicarbonate to correct metabolic acidosis.

Urea, in contrast, is a waste product that the dialysis process aims to remove from the blood. Including urea in the dialysate would defeat that purpose, because it would reduce the concentration gradient that drives diffusion of waste out of the blood and could even promote back-diffusion into the patient. Therefore, urea is not part of dialysate composition.