The malignant tertian malaria is caused by

The correct option is Plasmodium falciparum.

Malaria is a life-threatening disease caused by unicellular protozoan parasites belonging to the genus Plasmodium. These parasites are transmitted to humans through the bites of infected female Anopheles mosquitoes. Different species of Plasmodium cause different types of malaria, characterized by the severity of the disease and the periodicity of the fever spikes.

Among the species that infect humans:
1. Plasmodium falciparum: This is the most dangerous and virulent species. It is responsible for causing malignant tertian malaria (also known as falciparum malaria). It is characterized by severe complications, rapid progression, and high mortality rates if left untreated because it can cause clogging of capillaries (cerebral malaria). The term "tertian" refers to the recurrence of fever spikes typically every 48 hours (every third day, counting the day of the fever as day 1).
2. Plasmodium vivax: This species causes benign tertian malaria, which is less severe than malignant tertian malaria, although relapse can occur due to dormant liver stages (hypnozoites).
3. Plasmodium ovale: This species also causes a milder form of benign tertian malaria.
4. Plasmodium malariae: This species causes quartan malaria, where the fever spikes recur every 72 hours (every fourth day).

Therefore, malignant tertian malaria is specifically caused by Plasmodium falciparum.