Diarrhoea is the ______ leading cause of death in children under five years old.

$\text{Correct answer: A}$ 

$\text{Option (A):}$ 

$\bullet$ The first leading cause of death in children under 5 years is specific pathogens.

$\bullet$ More than 30% of the deaths occur due to the susceptibility of children to these pathogens.

$\bullet$ The second leading cause of death in children is diarrhoea. 

$\bullet$ It is caused due to loss of water with the faeces. It is the frequent passing of these loose watery feces that can slowly make the patient dehydrated.

$\bullet$ This generally occurs due to infections in the intestine and it can spread through unhygienic means. The presence of contaminated food can easily spread the pathogen.

Hence, the correct answer is option (A).