**Core Concept:** Inhalational anaesthesia is a method of inducing anaesthesia by inhaling volatile gases. It is commonly used in pediatric surgeries due to its advantages like rapid onset, smooth recovery and minimal interference with the cardiovascular system. The choice of anaesthetic agent depends on the patient's age, weight, and the surgery being performed.

**Why the Correct Answer is Right:** For a 6-month-old child with congestive cardiac failure due to patent ductus arteriosus (PDA), the choice of inhalational anaesthetic agent should be one that causes minimal haemodynamic changes and has a low propensity to worsen the pre-existing cardiac condition.

**Why Each Wrong Option is Incorrect:**

A. **Isoflurane**: Isoflurane is a commonly used inhalational anaesthetic agent but has a high affinity for myocardial receptors, causing a decrease in myocardial contractility and cardiac output. This could worsen the child's pre-existing congestive cardiac failure.

B. **Sevoflurane**: Sevoflurane is a commonly used anaesthetic agent, but it has a higher affinity for myocardial receptors than isoflurane, leading to a similar risk of worsening the child's cardiac condition.

C. **Desflurane**: Desflurane has a higher affinity for myocardial receptors compared to isoflurane and sevoflurane, increasing the risk of worsening the child's cardiac condition.

D. **Propofol**: Propofol is a commonly used intravenous anaesthetic agent for induction of anaesthesia. It is not the first choice for inducing anaesthesia in a child with PDA.

**Clinical Pearl:** In children with PDA, the choice of anaesthetic agent should be guided by the need to minimize haemodynamic changes and the potential to worsen the cardiac condition. Desflurane should be avoided due to its higher affinity for myocardial receptors. Isoflurane is a better choice than sevoflurane due to its lower affinity for myocardial receptors, but it should still be used with caution in a child with PDA. Propofol is not the first choice for induction of anaesthesia in a child with PDA.