A 56-year-old male with schizoaffective disorder and type 2 diabetes has been stable on ziprasidone 80 mg twice daily for two years. He presents for routine monitoring, and his ECG shows a QTc interval of 478 ms, increased from a baseline of 442 ms. His current medications include ziprasidone, metformin 1000 mg twice daily, and ondansetron 4 mg as needed for nausea, which he has been taking two to three times weekly for the past month due to gastroparesis. Serum electrolytes reveal potassium of 3.4 mEq/L and magnesium of 1.6 mg/dL. He is asymptomatic, denying palpitations, dizziness, or syncope. His psychiatric symptoms remain well-controlled. The PMHNP is evaluating the clinical significance of these ECG findings and potential contributing factors. Which of the following best represents the appropriate evaluation?