A 41-year-old male from Shanghai went on a business trip to Guangxi for 1 week in August. Two weeks after returning, he suddenly developed chills, severe rigors, followed by a temperature rise to 41°C within 30 minutes, accompanied by headache. The fever lasted 4-5 hours, followed by profuse sweating upon defervescence. After 3 such recurrent episodes, he sought medical attention, and malaria parasites were found on blood smear. Which stage of Plasmodium development in humans is associated with the patient's symptoms?