USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 121:

  Which of the following inhaled general anesthetic agents has an MAC that exceeds normal atmospheric pressure?

  A. enflurane

  B. halothane

  C. isoflurane

  D. nitrous oxide

  E. sevoflurane

  Correct Answer: D

  Section: Pharmacology The MAC value for nitrous oxide is 105110 % (i.e., 510% greater than atmospheric pressure), meaning that for nitrous oxide to be used as the sole anesthetic agent, hyperbaric conditions would have to be used to deliver both the nitrous oxide for anesthesia and oxygen needed for life. The other commonly used inhaled agents, including the halogenated hydrocarbons enflurane, isoflurane, halothane, and sevoflurane (choices A, B, C, and E) have MAC values of 6% or less. Although these data might be interpreted to imply that nitrous oxide is not useful in anesthesia, nitrous oxide is very valuable because of its very low toxicity and additive effects with more potent (but more toxic) agents. It is therefore commonly combined with other inhaled agents, especially the halogenated hydrocarbons. For example, 0.5% MAC of nitrous oxide plus 0.5% MAC of isoflurane delivers a full anesthetic concentration of gases to the lungs but requires <60% of the inspired air partial pressure, so oxygen can utilize the other 40%.

• Question 122:

  The most rapid onset of action of inhaled general anesthetics correlates with the smallest value for which of the following variables?

  A. blood-gas partition coefficient

  B. MAC

  C. molecular size

  D. oilwater partition coefficient

  E. onset of hepatic metabolism

  Correct Answer: A

  Section: Pharmacology The blood-gas partition coefficient is a measure of the solubility of the inhalation anesthetic in the blood relative to its solubility in the inspired air. Circulating blood provides the means of anesthetic delivery to the brain and the partial pressure determines the rate of transfer into the CNS. The solubility of an agent in blood determines how rapidly the partial pressure rises in the blood. Agents with high solubility (large blood- gas partition coefficients) require large amounts of the anesthetic to dissolve in the blood before the partial pressure in the blood increases enough to effectively deliver them to the brain. Thus, agents with lower blood solubilities (small blood-gas partition coefficients) have more rapid rates of onset of anesthesia. Desirable properties for inhalation anesthetic agents include high potency and low blood solubility. The halogenated hydrocarbons, such as desflurane and sevoflurane, fit these criteria and are used extensively. The MAC value (median alveolar anesthetic concentration) required for anesthesia (choice B) is a measure of the potency of the agent, but does not give an indication of the rate of onset of anesthesia. Molecular size (choice C) may play a role in determining the blood-gas partition coefficient, but it is only a partial determinant. The oilwater partition coefficient (choice D) is a measure of the lipid solubility of the anesthetic agent. This correlates with the potency as measured by the MAC. Hepatic metabolism (choice E) plays no role in onset of action, but may be important in terms of possible liver and kidney damage resulting from the production of toxic metabolites from some of the halogenated inhaled anesthetic agents.

• Question 123:

  A patient has completed a course of cancer chemotherapy and now has severe anemia, neutropenia, and thrombocytopenia. If only one intervention is possible, which of the following is the most appropriate therapy?

  A. epoetin (erythropoietin)

  B. filgrastim (G-CSF)

  C. growth hormone

  D. sargramostim (GM-CSF)

  E. testosterone

  Correct Answer: D

  Section: Pharmacology Apatient who is anemic, neutropenic, and thrombocytopenic requires stimulation of all three major cell lines in the bone marrow. The only drug currently available that accomplishes this broad-spectrum stimulant effect is sargramostim (granulocytemacrophage colony stimulating factor [GM-CSF]). Epoetin (choice A) is a more selective stimulant of erythrocyte production and is useful in simple anemia. Filgrastim (choice B) is a somewhat selective stimulant of leukocyte production and has much less effect on erythrocytes and platelet production than sargramostim. Growth hormone (choice C) and testosterone (choice E) have both been tried in the treatment of anemia with negligible success.

• Question 124:

  Which of the following agents produces vasodilation by increasing nitric oxide synthesis in endothelial cells?

  A. diazoxide

  B. histamine

  C. minoxidil

  D. nitroprusside

  E. verapamil

  Correct Answer: B

• Question 125:

  Which of the following statements correctly associates a CNS drug with its mechanism of action?

  A. bupropion--activation of endocannabinoid receptors

  B. diazepam--facilitation of GABAstimulated chloride channel opening

  C. fluoxetine--selective inhibition of presynaptic norepinephrine reuptake

  D. pentobarbital--inhibition of NMDA receptors

  E. tranylcypromine--inhibition of O-methylation of catecholamines

  Correct Answer: B

• Question 126:

  Which of the following agents is associated with numerous drug-drug interactions because of its inhibition of hepatic cytochrome P450 activity?

  A. atracurium

  B. cromolyn

  C. ketoconazole

  D. Phenobarbital

  E. rifampin

  Correct Answer: C

  Section: Pharmacology Ketoconazole is a potent inhibitor of cytochrome P450 isozymes; its antifungal action is the result of inhibition of fungal P450. Atracurium (choice A) is used to produce skeletal muscle relaxation during surgery. Atracurium is eliminated by a spontaneous chemical reaction and by plasma cholinesterases, and is not a substrate or inhibitor of cytochrome P450. Cromolyn sodium (choice B) is not a substrate or inhibitor of cytochrome P450. The extremely small amounts of this drug that are absorbed are excreted unchanged in the urine and bile. Phenobarbital (choice D) is eliminated by cytochrome P450 metabolism, and is an inducer of drug metabolism activity. Rifampin (choice E) similarly produces many drug interactions because of its ability to induce (rather than inhibit) cytochrome P450 activity.

• Question 127:

  Which of the following agents would cause the greatest increase in heart rate in a functioning transplanted heart?

  A. amphetamine

  B. cocaine

  C. ephedrine

  D. isoproterenol

  E. terbutaline

  Correct Answer: D

  Section: Pharmacology The transplanted heart lacks functional innervation for at least 2 years after surgery, and possibly longer. Because the nerves are cut in the procedure, nerve endings degenerate (Wallerian degeneration) and transmitter stores are lost. Adrenoreceptors on the denervated myocardial cells are normal or even increased in sensitivity, so responses to direct-acting betaadrenoreceptor agonists such as isoproterenol are retained. Indirectly acting sympathomimetics, on the other hand, are relatively ineffective (choices A, B, and C) because they act through the release or amplification of endogenous

• Question 128:

  Which of the following conditions is a primary indication for the use of esmolol?

  A. arrhythmia associated with thyroid storm

  B. chronic heart failure

  C. chronic hypertension

  D. diabetic nephropathy

  E. familial tremor

  Correct Answer: A

  Section: Pharmacology Esmolol is a very short-acting, parenteralonly beta-blocking drug. Transient arrhythmias such as those occurring during thyrotoxicosis are frequently responsive to the drug. Because the drug must be given parenterally and has duration of action of only 15 minutes, it is unsuited for any chronic condition (choices BE).

• Question 129:

  In a study of cardiovascular drugs, normal volunteers were given drug X in a small bolus dose while blood pressure and heart rate were recorded. After recovery, a long-acting blocking agent, Y, was administered. The direct effects of Y were not recorded. After drug Y had equilibrated with the tissues of the body, drug X was repeated in the same dosage. The results are shown in below figure.

  

  Drug Y behaves most like which of the following?

  A. atropine

  B. pralidoxime

  C. prazosin

  D. reserpine

  E. timolol

  Correct Answer: C

• Question 130:

  In a study of cardiovascular drugs, normal volunteers were given drug X in a small bolus dose while blood pressure and heart rate were recorded. After recovery, a long-acting blocking agent, Y, was administered. The direct effects of Y were not recorded. After drug Y had equilibrated with the tissues of the body, drug X was repeated in the same dosage. The results are shown in below figure.

  

  Drug X behaves most like which of the following?

  A. bethanechol

  B. epinephrine

  C. isoproterenol

  D. norepinephrine

  E. phenylephrine

  Correct Answer: D

  Section: Pharmacology Elevated blood pressure typically evokes a compensatory baroreceptor reflex with slowing of heart rate. Thus, the decrease in heart rate shown in the initial data can be interpreted as a reflex bradycardia. Bethanechol (choice A) is a muscarinic agonist and typically causes vasodilation, with a drop in blood pressure and a compensatory tachycardia. Epinephrine (choice B), an alpha-1-, alpha-2-, beta-1-, and beta-2-agonist, causes hypertension at high doses, but usually also causes tachycardia. Isoproterenol (choice C) is a beta-1-, beta-2- agonist, and does not cause hypertension or bradycardia. Norepinephrine (choice D) and phenylephrine (choice E) can both cause hypertension and reflex bradycardia. However, norepinephrine has beta-1-agonist action, whereas phenylephrine has only alpha effects. Thus, in the presence of an alpha-blocking agent, norepinephrine causes a beta-1-mediated tachycardia; phenylephrine has no effect on heart rate.