In the life cycle of Ascaris, the infective stage is

The correct answer is second larva (also known as the embryonated egg containing the second-stage rhabditiform larva).

Here is a step-by-step educational breakdown of why this is the correct option:

1. Understanding the Host and Life Cycle:
Ascaris lumbricoides is a giant intestinal roundworm of humans. Its life cycle is monogenetic, meaning it requires only a single host (humans) to complete its development.

2. Egg Production and Release:
Adult female worms in the human intestine lay eggs, which are passed out along with the feces. At this stage, the eggs are unsegmented and not yet infective to another human.

3. Development in the Environment:
Under favorable environmental conditions (moist soil, oxygen, and warm temperature), the fertilized egg undergoes cleavage.
The first division leads to the formation of the first-stage juvenile larva (first rhabditiform larva) inside the egg shell. This stage is not yet infective.
Within the egg shell, the first-stage larva undergoes its first molt (shedding of the outer cuticle) to transform into the second-stage rhabditiform larva.

4. The Infective Stage:
The egg containing this second larva is the fully embryonated egg. Only when a human accidentally ingests these embryonated eggs (via contaminated food, water, or soil) does infection occur. Thus, the second larva stage is the infective stage of Ascaris.