Ans. is 'a' i.e., Minimum alveolar concentration Minimal alveolar concentration (MAC)* Most important measure of potency is minimal alveolar concentration (MAC). MAC is the lowest concentration of the anesthetic in pulmonary alveoli needed to produce immobility in response to a painful stimulus (surgical incision) in 50% individuals. Higher the MAC, less potent the anesthetic agent.* Methoxyflurane has minimum MAC (0.16%) - The most potent inhalational agent.* N2O has maximum MAC (105) - The least potent inhalational agent.* Order of potency in decreasing order (MAC in increasing order): -* Methoxyflurane (MAC = 0.16 %) > Trilene (MAC = 0.2%) > Halothane (MAC = 0.74%) > Chloroform (MAC = 0.8 %) > Isoflurane (MAC = 1.15 %) > Enflurane (MAC = 1.68%) > Ether (MAC = 1.92 %) > Sevoflurane (MAC = 2.0 %)> Desflurane (MAC = 6.0%) > Cyclopropane (MAC = 9.2%) > N20 (MAC 104%).Factor affecting MAC1. Age: - Young age increases and old age decreases MAC.2. Alcohol: - Chronic intoxication increases and acute intoxication decreases MAC.3. Temperature: - Both hypothermia and hyperthermia decrease MAC.4. Electrolyte: - Hypercalcemia, Hypermagnesemia & Hyponatremia decrease. Whereas hypernatremia increases MAC.5. Anemia : - Decrease MAC6. Hypoxia (pO2 < 40), and hypercarbia (pCO2 > 95) decrease MAC.7. Pregnancy: - Decreases MAC8. Drugs: -A. Decreasing MAC - Local anaesthetics (except cocaine) Opioids, Ketamine, Barbiturates, Benzodiazepines, Verapamil, Lithium, Sympatholytics (Methyldopa, reserpine, Clonidine, Dexmedetomidine), Chronic amphetamine use.B. Increasing MAC: - Acute amphetamine intake, Cocaine, ephedrine.