Dialysis fluid lacks large molecules like plasma proteins

Steps:

• Dialysis uses a semi-permeable membrane allowing small molecules to diffuse between blood and dialysate.
• Dialysate composition includes glucose and salts to match plasma but excludes proteins and initially lacks urea.
• During treatment, urea diffuses into dialysate, while proteins remain in blood.
• Sampling dialysate reveals small solutes but no proteins due to membrane size cutoff.

Why C is correct:

• Plasma proteins exceed the membrane's pore size (typically >60 kDa), preventing diffusion per dialysis principles.

Why the others are wrong:

• A: Glucose is intentionally added to dialysate to maintain blood glucose levels.
• B: Salt ions are included in dialysate to equilibrate electrolyte concentrations with blood.
• D: Urea diffuses from blood into dialysate as a waste removal mechanism.

Final answer: C