Which of the Following Statements Best Captures the Essence of a Second

Which of the following statements best captures the essence of a second messenger in the mechanisms of the endocrine system?

A) Second messengers act as the intracellular signal that responds to the presence of a hormone.
B) Endocrine-producing cells must release both a hormone and a second messenger in order to exert a distant effect.
C) Second messengers act to supplement hormone effects on cell receptors when the desired hormonal effect must be either increased.
D) Second messengers provide an alternative pathway for endocrine effects on a cell that bypass the normal receptor pathways.