What change should be made to a patient's antibiotic regimen to decrease the risk of torsades de pointes if their QT interval increases significantly?

Switching from azithromycin to doxycycline is a sound choice for decreasing the risk of torsades de pointes in a patient whose QT interval has increased significantly. Azithromycin is known to potentially prolong the QT interval, and in patients who are already at risk of developing arrhythmias, this increase can lead to serious complications like torsades de pointes.

Doxycycline, on the other hand, does not have a significant effect on the QT interval and is a safer alternative for treating infections without posing additional cardiovascular risks in this context. This switch, therefore, helps to manage the underlying infection while also addressing the concern of QT prolongation effectively.

The other options do not adequately address the risk associated with QT interval prolongation. For instance, continuing the current regimen does not mitigate the risk, and switching to a single agent like moxifloxacin or changing to a combination regimen for health care-acquired pneumonia may not specifically target the problem of QT prolongation linked with the antibiotics being used.