A 58-year-old man with chronic kidney disease (eGFR 22 mL/min/1.73m²) presents to the emergency department with dyspnea and altered mental status. His wife reports he has been increasingly lethargic over the past 3 days. On examination, he is tachypneic (RR 28/min) with Kussmaul respirations. Vital signs: BP 128/82 mmHg, HR 102/min, T 36.8°C. Laboratory investigations reveal: - pH: 7.18 - PaCO₂: 28 mmHg - HCO₃⁻: 11 mEq/L - Na⁺: 138 mEq/L - K⁺: 6.2 mEq/L - Cl⁻: 104 mEq/L - BUN: 68 mg/dL - Creatinine: 4.8 mg/dL - Anion gap: 23 mEq/L What is the primary acid-base disorder?