A 2-year-old girl with known Tetralogy of Fallot presents to the emergency department with severe cyanosis, loss of consciousness, and convulsions. Her SpO₂ is 45% despite high-flow oxygen. The parents report she had a prolonged crying episode 30 minutes ago. On examination, she has a harsh systolic murmur at the left sternal border and a single loud S2. Arterial blood gas shows pH 7.18, PaCO₂ 32 mmHg, HCO₃⁻ 12 mEq/L, and PaO₂ 38 mmHg. What is the most likely cause of her acute deterioration?