Dialysis membrane filters by molecular size

Steps:

• Dialysis membranes have pores that allow small molecules (<500 Da) to diffuse freely while blocking larger ones.
• Compare molecular weights: urea (60 Da), sodium ions (~23 Da), glucose (180 Da) are small; insulin (5,800 Da) is a large peptide hormone.
• In hemodialysis, the membrane selectively removes waste like urea via diffusion down concentration gradients.
• Insulin, being too large, remains in the blood and cannot pass through.

Why B is correct:

• Insulin is a protein with molecular weight ~5,800 Da, exceeding the membrane's pore size cutoff (~500 Da) for diffusion.

Why the others are wrong:

• A: Glucose (180 Da) is small enough to diffuse through the pores.
• C: Sodium ions (~23 Da) easily pass as electrolytes in solution.
• D: Urea (60 Da) is the primary waste targeted for diffusion removal.

Final answer: B