Ans. a (P. vivax) (Ref. Harrison 18th/ ch. 210)# After entry into the bloodstream, merozoites rapidly invade erythrocytes and become trophozoites.# Attachment is mediated via a specific erythrocyte surface receptor.# In the case of P. vivax, this receptor is related to the Duffy blood-group antigen Fya or Fyb.# Most West Africans and people with origins in that region carry the Duffy-negative FyFy phenotype and are therefore resistant to P. vivax malaria.ETIOLOGY AND PATHOGENESIS OF MALARIA# Four species of the genus Plasmodium cause nearly all malarial infections in humans (although rare infections involve species normally affecting other primates). These are:- P. falciparum,- P. vivax,- P ovale, and- P malariae.# Almost all deaths are caused by falciparum malaria. Human infection begins when a female anopheline mosquito inoculates plasmodial sporozoites from its salivary gland during a blood meal.# By amplification process (known as intrahepatic or preerythrocytic schizogony or merogony), a single sporozoite eventually may produce 10,000 to >30,000 daughter merozoites.# The swollen liver cell eventually bursts, discharging motile merozoites into the bloodstream.# These then invade the RBCs and multiply 6- to 20-fold every 48 to 72 h.# When the parasites reach densities of ~50/mL of blood, the symptomatic stage of the infection begins.# The dormant forms, or hypnozoites, are the cause of the relapses that characterize infection with P. vivax and P. ovale,# By the end of the 48-h intraerythrocytic life cycle (72 h for P. malariae), the parasite has consumed nearly all the hemoglobin and grown to occupy most of the RBC. It is now called a schizont.# Multiple nuclear divisions have taken place (shizogony or merogony), and the RBC ruptures to release 6 to 30 daughter merozoites, each potentially capable of invading a new RBC and repeating the cycle.# When a female Anopheline mosquito takes a blood meal, the male and female gametocytes continue their sexual development.# Schizonts are rarely seen in the peripheral blood and their presence may indicate a potentially serious parasitemia.