Review Question - QID 109857

A 20-year-old premedical student travels abroad for a global health service semester and presents to the local clinic with palpitations. She initially dismissed these symptoms as a side effect of working too hard; however, she has been noticing that these episodes have increased in frequency. Otherwise, she has had headaches and episodes of sweating, but she says that many members of her family have migraines. In the clinic, her temperature is 99°F (37°C), blood pressure is 170/120 mmHg, pulse is 105/min, respirations are 20/min. Other laboratory testing is not available. The doctor provides an older medication that he says does not bind to the relevant receptor but instead blocks an upstream process. The drug provided most likely has which of the following mechanisms of action?