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MCAT Question A Day - 6/20/14 - Answer!

Aldosterone exerts its effects on target cells by:

A.  binding to a receptor at the cell surface, setting off a second-messenger cascade.
B.  diffusing into adrenal cortical cells. where it influences transcription of certain DNA sequences.
C.  flowing across the synapse, where it binds and initiates an action potential.
D.  entering into target cells, where it increases the rate of production of sodium-potassium pump proteins.

D is correct. Aldosterone, as we can tell by its name, is a steroid (any hormone whose name ends in "sterone" or something similar is a steroid). This allows us to eliminate choice A, because steroid hormones do not need cell membrane receptors or second-messenger systems. They simply diffuse across the cell membrane. We can eliminate B because the adrenal cortex is aldosterone's source, not its target tissue. C describes events at a synapse. Aldosterone actually exerts its effect by doing what D says, increasing the production of sodium-potassium pump proteins.