While migrating larva of Ascaris takes place its 4th moult in:

Question

Accepted Answer

The correct option is 4: Intestine

The life cycle of the human roundworm, *Ascaris lumbricoides*, involves a complex and obligatory migratory path through the host's body, which is essential for the larva to mature. The process begins when an infective egg is ingested, and the second-stage larva (L2) hatches in the small intestine. Following this, the L2 larva penetrates the intestinal wall and begins its journey through the circulatory system (intestine $\rightarrow$ liver $\rightarrow$ heart $\rightarrow$ lungs). The larvae undergo their second and third moults (L2 to L3, and L3 to L4) after reaching the alveoli of the lungs, where they grow significantly. After ascending the respiratory passages and being swallowed, the now-L4 larva returns to the $\textbf{small intestine}$. It is upon this re-entry, its definitive habitat, that the L4 larva performs its **fourth and final moult**, transforming into the young adult worm (L5 stage). The small intestine, therefore, serves as the site of the first larval emergence and the final, fourth moult that precedes sexual maturity and reproduction.

The incorrect option is 1:
The **Liver** is merely a transit point in the migratory route of the *Ascaris* larva. After penetrating the intestinal mucosa, the L2 larvae are carried to the liver via the hepatic portal vein, making it the first organ encountered. They typically remain in the liver for only a few days before being carried by the blood to the heart and then the lungs. No ecdysis (moulting) or significant developmental progression is known to occur while the larvae are passing through the liver.

The incorrect option is 2:
The **Lung** is a critical developmental site, but it hosts the intermediate moults, not the fourth one. After being pumped from the right side of the heart, the larvae enter the pulmonary circulation and penetrate the lung alveoli. It is in the lungs that the larvae undergo the second moult (L2 to L3) and the third moult (L3 to L4), coupled with a period of rapid growth. The L4 larva then moves out of the lungs to be swallowed, indicating that its final, fourth moult has not yet taken place.

The incorrect option is 3:
The **Heart** is purely a channel for the *Ascaris* larva during its migration. The larvae pass through the right side of the heart—specifically the right atrium and right ventricle—as they are transported from the liver (via the inferior vena cava) to the lungs (via the pulmonary artery). The heart’s function in this context is strictly to pump the blood, and thus the larvae, toward the lung capillaries. The larvae do not settle, grow, or undergo any of the required moulting processes while in the heart chambers.