A 56-year-old male was admitted with oliguria for 1 week. His blood pressure (BP) was 180/120 mmHg, with somnolence, anemia, facial and bilateral lower extremity edema. Lab results: blood urea nitrogen (BUN) 42 mmol/L, creatinine (Cr) 1380 μmol/L, serum potassium (K) 6.2 mmol/L, serum calcium (Ca) 2.0 mmol/L, CO2 combining power 12 mmol/L. During correction of acidosis, he suddenly developed tetany but remained alert and oriented, with no pathological signs. The most likely cause of tetany is: