**Core Concept:** Diarrhoea is a medical condition characterized by the passage of loose, watery stools, and it can be caused by various factors such as infections, medications, or dietary issues. Rapid and deep breathing is a clinical sign of increased carbon dioxide levels in the blood, which is known as respiratory alkalosis.

**Why the Correct Answer is Right:** In a child with diarrhoea, rapid and deep breathing could be due to the loss of bicarbonate ions (HCO3-) from the gastrointestinal tract during diarrhoea, leading to a decrease in blood pH and an increase in the partial pressure of carbon dioxide (PaCO2). This results in the kidneys producing more aldosterone, which in turn promotes sodium and water reabsorption in the distal renal tubules, leading to dehydration and increased carbon dioxide levels.

**Why Each Wrong Option is Incorrect:**

A. Hypoventilation (inadequate breathing) can cause respiratory acidosis (increased PaCO2 and decreased pH), not alkalosis.
B. Respiratory alkalosis (increased PaCO2 and decreased pH) can be caused by excessive ventilation or carbon dioxide removal, but this is not the case here.
C. Respiratory alkalosis can be caused by excessive carbon dioxide retention due to lung or respiratory muscle dysfunction, but this option is incorrect because the correct answer explains the mechanism of respiratory alkalosis in diarrhoea.
D. Respiratory alkalosis can occur with excessive use of certain medications like albuterol, which increases the respiratory rate, but this is not the case in this scenario where the cause is diarrhoea-induced respiratory alkalosis.

**Clinical Pearl:** Respiratory alkalosis in diarrhoea should prompt a thorough evaluation for the underlying cause and appropriate management, including fluid and electrolyte replenishment. It is essential to differentiate between respiratory alkalosis due to diarrhoea and other causes, such as medications, lung or respiratory muscle dysfunction, and respiratory alkalosis due to other conditions like asthma, chronic obstructive pulmonary disease (COPD), or asthma exacerbation.

**Correct Answer:** D. Respiratory alkalosis (increased PaCO2 and decreased pH)

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**Core Concept:** Diarrhoea can lead to respiratory alkalosis due to the loss of bicarbonate ions (HCO3-) in the gastrointestinal tract, resulting in a decrease in blood pH.

**Why the Correct Answer is Right:** In diarrhoea, bicarbonate ions (HCO3-) are lost in the stool, leading to a decrease in blood pH. This compensatory response is characterized by an increased respiratory rate and depth to remove the excess CO2 from the body, which results in respiratory alkalosis.

**Why Each Wrong Option is Incorrect:**

A. Hypoventilation (decreased ventilation) causes respiratory acidosis (increased PaCO2 and decreased pH) and is not the correct answer to the question.
B. Hypokalaemia (low potassium levels) can cause tachypnoea (increased respiratory rate), but