Ans. b (50%). (Ref. Shorttext book, Anaesthesia, 2nd ed., 54-55)Anesthetic potency of volatile anesthetics is measured by MAC. MAC represents the alveolar concentration of an anesthetic (at one atmosphere) that prevents movement in 50 percent of the subjects in response to pain.MINIMUM ALVEOLAR CONCENTRATION (MAC) - MAC = 1/ potency of drug# For determination of MAC in humans, the usual stimulus used is surgical skin incision.# It represents after a short time the partial pressure of the anesthetic in the central nervous system (CNS) and it is therefore the most useful index of anesthetic potency.# MAC is age-dependent, being lowest in newborns, reaching a peak in infants, and then decreasing progressively with increasing age.# The MAC can also be altered following administration of opioids.# As a rule of thumb, the addition of every one percent of alveolar nitrous oxide to another inhalation anesthetic will decrease in the MAC of that gas about one percent.# Increases in MAC result from hyperthermia and hypernatremia.# Decreases in MAC can result from hypothermia, hyponatremia, pregnancy, hypotension, and drugs such as lithium, lidocaine, opioids, and alpha2 agonists.Minimum alveolar concentration of inhaled anesthetics in 100% oxygen:- Halothane 0.74 percent- Enflurane 1.68 percent- Isoflurane 1.15 percent- Desflurane 6.3 percent- Sevoflurane 2.0 percent- Nitrous oxide 104 percent Factors increasing MACFactors decreasing MACDrugs decreasing MAC1) Hyperthermia2) MAO inhibitors3) Hypernatremia4) Alcohol abuse (chronic)1) Hyperthermia2) Hypoxia3) Metabolic acidosis4) Pregnancy5) Alcohol abuse (acute).1) Opiods, Pancuronium2) Propofol, Ketamine3) Benzodiazepines4) Barbiturates, Lidocaine5) Clonidine.