After extraction, patient turns blue and tired (cyanotic and fatigued). What is the most likely cause?

CYANOSIS (blue coloration) + fatigue/drowsiness AFTER nitrous oxide sedation indicates HYPOXIA — insufficient oxygen delivery. This can occur if: nitrous concentration is too high, oxygen flow is too low, patient is not receiving adequate oxygenation, or if N2O diffusion hypoxia occurs at the end (when N2O leaves pulmonary capillaries rapidly and dilutes alveolar oxygen). Treatment: 100% oxygen. Vasovagal syncope (B) causes pallor, not cyanosis. A is correct.