A 76-year-old male with diabetes mellitus (DM). Over the past month, he frequently experienced chest tightness, dyspnea, and paroxysmal nocturnal dyspnea. The family found him in a coma and rushed him to the hospital. His daily medications include gliclazide (3 tablets) and phenformin (3 tablets), with satisfactory blood glucose control. Examination: deep coma, deep and rapid breathing, BP 60/40 mmHg, HR 124 bpm, urine glucose (++), urine ketone (±), blood glucose 12.4 mmol/L, pH 6.9, serum potassium (K) 4.9 mmol/L, anion gap 15, ECG shows sinus tachycardia. The most likely cause is: