Malignant tertian/subtertian fever is caused by
Plasmodium falciparum
Malignant tertian or sub tertian fever is caused by Plasmodium falciparum in which the cold stage is less pronounced and the fever stage is more prolonged and intensified (if the fever is recurring it occurs every 2nd day). However, the fever is usually continuous or only briefly remittent. There is no chills stage. This type of malaria is more dangerous because of the complications caused by the capillary blockage. Large numbers of erythrocytes are parasitized and destroyed, which may result in dark-coloured urine. So, the correct answer is 'Plasmodium falciparum'.