Question

In cycle of Ascaris lumbricoides, rhabditiform larva undergoes 2nd and 3rd moultings in
A. Liver
B. Heart
C. Alveoli of lungs
D. Small intestine

Answer 1

Hint: Fertilized eggs of Ascaris are passed from the host's body with its faeces into the soil where at first place larva moulting takes place, larva gets moulted into the second stage larvae which infects the host through the consumption of contaminated food and water.

Complete answer:
Ascaris lumbricoides is a roundworm which infects humans. This is responsible for the disease ascariasis. This infects human beings via a fecal-oral route. Adult females release eggs and these eggs are shed in the feces. Unfertilised eggs never become infective and these are mostly seen in the fecal samples. Fertilised eggs become infective after 18 days to many weeks in the soil depending upon the conditions of the environment like most, warm or shaded soil.

So, when this fertilised egg is ingested then the Rhabditiform larva gets hatched and it then penetrates into the wall of gastrointestinal tract and from there it enters the blood stream. Now it is carried from there to the heart and the liver and afterwards it enters the pulmonary circulation to break free in the alveoli where it grows and moults. Here, the first larva moulting takes place in the liver and heart and this larva does not have the power of infection yet.

This becomes moulted into second stage larvae which then infects the host through the consumption of contaminated food and water. It undergoes the 2nd moulting to produce the 3rd larva stage in the alveoli of lungs. In the three weeks thus larva passes from the respiratory system and reaches the small intestine where it matures into an adult male or female worm.

If we look at options now, in the liver and heart, larva undergoes first moulting and it undergoes 2nd and 3rd moulting in alveoli of lungs. And lastly in three weeks it passes from the respiratory system and reaches the small intestine. It proves that larva undergoes 2nd and 3rd moultings in the alveoli of lungs.

Hence, the correct answer is option (C).

Note: 2nd and 3rd moulting occurs in the alveoli of lungs as from the first moulting in liver and heart it enters the pulmonary circulation to break free in the alveoli where it grows and moults.