A 39-year-old male on methadone maintenance therapy at 120 mg daily for opioid use disorder is evaluated after developing recurrent syncopal episodes. An electrocardiogram reveals a QTc interval of 542 ms, increased from his baseline of 448 ms six months ago. Review of his medication changes reveals that fluconazole 200 mg daily was started three weeks ago for a persistent fungal infection. His other medications include gabapentin 600 mg three times daily and melatonin 5 mg at bedtime. Which of the following best explains the relationship between the medication change and the electrocardiographic finding?