A 63-year-old male patient presents with a 10-minute episode of tetany and loss of consciousness, which has now ceased. Past medical history includes diabetes mellitus (DM), hypertension (HTN), and sleep apnea syndrome. According to family, the patient uses non-invasive ventilation at night to improve breathing, but the ventilator was damaged and not repaired 1 day ago, leading to discontinuation. On examination: lethargy, poor mental status, coherent speech, no cyanosis, no crackles or wheezes on lung auscultation, abdominal obesity, pitting edema in both lower limbs, non-palpable pedal pulses, and negative Babinski sign. Which test should be performed first?