## **Core Concept**
The infectivity of microfilariae to a vector, typically a mosquito in the case of filarial parasites, depends on the density of microfilariae in the host's peripheral blood. Microfilariae are the larval stages of filarial parasites that circulate in the bloodstream.

## **Why the Correct Answer is Right**
The density of microfilariae in the blood is crucial for determining the infectivity to the vector. A threshold of **> 5 microfilariae per 20 mm^3 (or per microliter)** of blood is generally considered necessary for the blood to be infective to the vector. This density ensures that when a mosquito ingests blood during a feed, it takes in enough microfilariae to potentially lead to the development of infective third-stage larvae within the mosquito.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a much lower threshold, which might not be sufficient for infectivity.
- **Option B:** Similarly, this option presents another lower threshold that may not guarantee infectivity to the vector.
- **Option D:** This option implies a much higher density than necessary for infectivity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the microfilarial density in blood can vary diurnally (depending on the time of day) and is influenced by the species of filarial parasite. For *Wuchereria bancrofti*, the most common cause of lymphatic filariasis, microfilariae are typically more abundant in the peripheral blood at night, a phenomenon known as nocturnal periodicity.

## **Correct Answer:** C. > 5 microfilariae per 20 mm^3.