Why do only some cells respond to a circulating hormone even though the hormone is present throughout the bloodstream?

• A. Only cells that express the specific receptor for that hormone can bind it and transduce the signal, so other cells are unresponsive despite exposure.
• B. All cells respond but only target cells show visible changes because they have more abundant mitochondria to carry out the hormone's action.
• C. Hormones are rapidly degraded in all tissues except their target cells, which is why only target cells are exposed to functional hormone concentrations.
• D. The blood-brain barrier prevents hormones from reaching any peripheral tissue except the hypothalamus, so peripheral responses are mediated by neurotransmitters only.

Answer: (A)