Which species is characterized by a trophozoite with dark coarse pigment stretching across a normal-sized red blood cell?

• A. Plasmodium falciparum
• B. Plasmodium ovale
• C. Plasmodium vivax
• D. Plasmodium malariae

Answer: (D)

The feature being tested is how to identify malaria species on a blood smear by the appearance of the trophozoite and the red blood cell it inhabits. The description of a trophozoite with dark, coarse pigment stretching across a normal-sized red blood cell points to Plasmodium malariae. In malariae, the pigment (hemozoin) tends to be dark and coarse and can appear as a band or stripe through the cytoplasm of the trophozoite, while the host red blood cell remains normal in size. This combination—normal RBC size with a trophozoite containing dark, coarse pigment spanning the cell—is characteristic of malariae.

In contrast, the other species show different cues. Plasmodium falciparum typically presents with small, delicate ring forms and sometimes multiple infections per RBC, but pigment is not a defining, spanning feature. Plasmodium vivax and Plasmodium ovale both enlarge red blood cells and have trophozoites that are more amoeboid with Schuffner’s dots; their key distinguishing clue is the enlarged RBCs, not dark pigment stretching across a normal cell.