**Question:** Infectious stage of plasmodium is:

A. gametocyte
B. schizont
C. ring form
D. trophozoite

**Core Concept:**
Plasmodium is a genus of protozoa causing malaria in humans. The life cycle of Plasmodium involves both asexual and sexual stages. The infectious stage refers to the sexual stage, which is crucial for transmission to a new host.

**Correct Answer: A. gametocyte**

**Why the Correct Answer is Right:** The correct answer is gametocyte because it is the sexual stage of Plasmodium that is responsible for the transmission of the parasite to a new host, specifically a female Anopheles mosquito. In the mosquito vector, the gametocytes develop into male and female gametes, which fuse to form a zygote. This zygote develops into an ookinete, which penetrates the mosquito's gut wall and forms an oocyst. Inside the oocyst, sporozoites are produced, which can penetrate human skin upon mosquito bite and initiate the asexual stage of the parasite, leading to the development of the disease in the human host.

**Why Each Wrong Option is Incorrect:**

**A. gametocyte (incorrect)**: As mentioned above, gametocytes are the sexual stage involved in transmission, but they themselves do not cause disease in the human host.

**B. schizont (incorrect)**: Schizont is an asexual stage of Plasmodium, where the parasite undergoes division and multiplication, not involved in transmission.

**C. ring form (incorrect)**: Ring form is an asexual stage of Plasmodium, which is responsible for the clinical manifestations of malaria in the human host, not involved in transmission.

**D. trophozoite (incorrect)**: Trophozoite is an asexual stage of Plasmodium, where the parasite multiplies within the host erythrocytes, not involved in transmission.

**Clinical Pearl:** Understanding the life cycle of Plasmodium is crucial for diagnosing and treating malaria. The correct stages involved in transmission are gametocytes, which differentiate into male and female gametes in the mosquito, and eventually produce sporozoites that cause the disease in humans after the mosquito bite.