Potassium sparing diuretics

A patient with newly diagnosed heart failure is started on a number of medications, including an ACE inhibitor, beta blocker, loop diuretic, and eplerenone. The mechanism of action of eplerenone is best described as:

1: Inhibition of a luminal enzyme
2: Inhibition of an apical cotransport protein
3: Inhibition of an apical ion channel
4: Inhibition of an intracellular receptor