A 30-year-old Nigerian woman presents to a UK hospital with fever 18 days after returning from visiting family in Kano, Nigeria. She took atovaquone/proguanil prophylaxis but missed the final 3 days of the post-travel course. Blood film shows P. falciparum at 0.3% parasitaemia. She has no features of severe disease. What is the most likely reason for breakthrough despite prophylaxis?