Okay, let's tackle this question. The question is about which form of the malarial parasite is present in the saliva of an infective mosquito. Hmm, malarial parasites have a complex life cycle, right? So I need to recall the different stages and where they occur.

First, the malarial parasite is transmitted through the bite of an infected Anopheles mosquito. When the mosquito bites a human, it injects saliva to prevent blood clotting, and that's when the parasite is transmitted. The key here is to identify the specific form present in the saliva.

I remember that the life cycle includes sporozoites, merozoites, trophozoites, and gametocytes. Let me think. The mosquito takes up gametocytes when it feeds on an infected human. Inside the mosquito, gametocytes develop into sporozoites. When the mosquito bites another human, it injects sporozoites into the bloodstream. So the form in the saliva should be sporozoites.

Wait, the options are A to D, but the correct answer is not listed here. The user provided the correct answer as ., which I assume is a placeholder. Let me confirm. The correct answer is Sporozoites. So the options must include that. The other forms like gametocytes are present in human blood, not in the mosquito's saliva. Merozoites are released in the liver or blood, and trophozoites are in red blood cells. So the correct answer is Sporozoites. The clinical pearl here is that sporozoites are the infective stage transmitted by the mosquito, which is crucial for the life cycle. Got it.

**Core Concept**
The malarial parasite *Plasmodium* has a life cycle involving both human and mosquito hosts. Transmission occurs when a mosquito injects **sporozoites** into the human bloodstream during a bite. This stage is critical for initiating infection in the human host.

**Why the Correct Answer is Right**
Sporozoites are the infective stage of *Plasmodium* transmitted by mosquitoes. After a mosquito ingests gametocytes from an infected human, sexual reproduction occurs in the mosquito gut, forming zygotes that develop into ookinetes and oocysts. Sporozoites are released into the mosquito’s hemocoel and migrate to the salivary glands. When the mosquito bites, sporozoites are injected into the human host, initiating infection. This is the only stage present in mosquito saliva.

**Why Each Wrong Option is Incorrect**
**Option A:** Gametocytes are present in human blood and ingested by mosquitoes during a blood meal, not in mosquito saliva.
**Option B:** Merozoites are released in the human liver or bloodstream after sporozoites invade hepatocytes or red blood cells.
**Option C:** Trophozoites develop from merozoites inside red blood cells and are not transmitted by mosquitoes.

**Clinical Pearl / High-Yield Fact**
Remember the life cycle: **Sporozoites → Liver → Merozoites → Red blood cells → Gametocytes**. Sporozoites are the key link between mosquito and human hosts. Confusion often arises between sporozoites (mosquito-transmitted) and gametocytes (human-to-mosquito).

**Correct Answer: D. Sporozoites

✓ Correct Answer: D. Sporozoite