A 62-year-old female with generalized anxiety disorder stabilized on buspirone 15 mg twice daily reports that she recently began drinking 16 ounces of grapefruit juice each morning as part of a new dietary regimen. Over the following two weeks, she notices increased drowsiness, dizziness, and lightheadedness that were not present before the dietary change. Her other medications include amlodipine 5 mg daily and a multivitamin. Which of the following best explains the pharmacokinetic mechanism by which grapefruit juice consumption could produce these symptoms?