**Question:** Minimum Alveolar concentration of halothane is

A. 0.5%
B. 0.7%
C. 1%
D. 2%

**Core Concept:**
Halothane is a volatile anaesthetic agent used for general anaesthesia induction and maintenance. It acts by binding to the alpha-subunit of the GABAA receptor, which is involved in the inhibition of neuronal excitability. This results in depressed consciousness, muscle relaxation, and cardiovascular stability during surgery.

**Why the Correct Answer is Right:**
The minimum alveolar concentration (MAC) of halothane is the lowest concentration of anaesthetic required to prevent a surgical stimulus from evoking a motor response in 50% of patients. The MAC of halothane is influenced by factors like body weight, age, and respiratory function. In adults, the MAC of halothane is typically around 0.5%. Therefore, option A (0.5%) is the correct answer.

**Why Each Wrong Option is Incorrect:**
Option B (0.7%) is too high, as it exceeds the typical MAC value for halothane in adults. Option C (1%) is also too high, as the MAC value is usually expressed as a percentage. Option D (2%) is excessively high, potentially causing significant side effects and hemodynamic instability.

**Clinical Pearl:**
The MAC of different anaesthetic agents varies between individuals and conditions, making it essential for anaesthesiologists to titrate the anaesthetic concentration based on the patient's characteristics, depth of anaesthesia, and surgical requirements.

**Correct Answer:** A. 0.5%