Question

The graph shows the relative filterability of molecules across the glomerular capillary wall plotted against effective molecular radius, for three curves A, B and C carrying different charges. Based on charge, which molecule is filtered most readily?
(Refer to the figure; for a given molecular radius, filterability order A > B > C, with C marked as anions.)

• A. A (cation)
• B. B (neutral)
• C. C (anion)
• D. All are filtered equally

Hint:

The filtration barrier is negatively charged, so it lets positive molecules through most easily.

Answer 1

The Correct Option is A

The plot is a classic renal-physiology graph of relative filterability versus effective molecular radius for three differently charged species.

Underlying principle - the glomerular filtration membrane is lined with fixed negative charges (heparan sulfate and sialic-acid-rich glycoproteins). By simple electrostatics:
• Like charges repel to anions are held back.
• Opposite charges attract to cations pass most easily.

Hence, holding size constant, the ranking is:
$$\text{filterability}_{cation} > \text{filterability}_{neutral} > \text{filterability}_{anion}$$

On the figure, curve A sits highest (cation), B is intermediate (neutral) and C is lowest and labelled 'anions'. The molecule filtered most readily on the basis of charge is therefore curve A, the cation.

This same charge barrier explains why anionic albumin is largely retained in health, and why its loss (as in minimal change disease) produces selective proteinuria.

\[\boxed{\text{A (cation)}}\]