Which antibiotic is most likely to further prolong the QT interval in a patient with long QT syndrome?

Clarithromycin is known to potentially prolong the QT interval, especially in patients who already have long QT syndrome. This antibiotic belongs to the macrolide class, which has been shown to interact with cardiac ion channels, particularly affecting the potassium channels involved in repolarization. This prolongation can lead to an increased risk of torsades de pointes, a life-threatening form of polymorphic ventricular tachycardia.

In contrast, the other antibiotics listed—amoxicillin, ceftriaxone, and doxycycline—are not typically associated with significant effects on the QT interval. Amoxicillin and ceftriaxone are beta-lactam antibiotics that do not have notable effects on cardiac repolarization. Doxycycline, a tetracycline, also lacks the association with QT interval prolongation that clarithromycin has demonstrated. Therefore, in a patient with long QT syndrome, clarithromycin would be the antibiotic most likely to exacerbate their condition.