**Core Concept**
Dehydration in children with diarrhoea is a common condition that requires prompt assessment and management. The assessment involves evaluating the severity of dehydration, which can be classified into three categories: mild, moderate, and severe. The presence or absence of thirst and tears is an important clinical indicator of the severity of dehydration.

**Why the Correct Answer is Right**
The absence of tears in a child with dehydration indicates a severe level of dehydration, as tears are a vital component of the body's fluid balance. When a child is severely dehydrated, the body's priority is to maintain blood pressure and perfusion of vital organs, and as a result, the production of tears is significantly reduced. This is because the body redirects fluid from non-essential areas, such as the eyes, to vital organs. In contrast, the presence of thirst indicates a mild to moderate level of dehydration, as the body is still able to sense the need for fluid replacement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of thirst is an indicator of mild to moderate dehydration, not severe dehydration.
**Option B:** This option is incorrect because the absence of thirst is not a reliable indicator of dehydration severity, as some children may not exhibit thirst even in the presence of dehydration.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the assessment of dehydration in children involves a combination of clinical indicators, including the level of thirst, tears, and urine output. A child with severe dehydration may exhibit a decreased level of consciousness, tachycardia, and poor peripheral perfusion.

**Correct Answer: D. Severe dehydration**