After an erythrocyte is penetrated by a merozoite, the parasite loses its penetration organelles and begins feeding and growing as a trophozoite. In the young trophozoite, the large central vacuole surrounded by the thin band of cytoplasm, and single chromatin dot on the periphery, give it the appearance of a "signet ring" or "pinky ring", hence the common name "ring stage."
This photo shows a ring stage trophozoite of P. vivax within an erythrocyte. In older infections the host cell becomes stippled with fine red granules called Schuffner's dots, which can serve to distinguish this species from some other species, but they are not visible in these young trophozoites.