USMLE Step 1 - Problem Drill 14: Pharmacology

Transcription

1 USMLE Step 1 - Problem Drill 14: Pharmacology Question No. 1 of shift dose response curves to the right, decrease potency and increase the EC50. Question #01 (A) Receptors (B) Non-competitive antagonists (C) Agonists (D) Irreversible antagonists (E) Competitive antagonists Receptors are the site of action of drugs and enzymes. They are the components of a cell or organisms that interact with a drug that initiate the chain of events leading to drugs observed effects. B. Incorrect! Non-competitive antagonists decrease efficacy of the agonist and shift the doseresponse curves downward. Agonists initiate changes in cell function via acting at receptors. They have a specified potency, which is the range of doses over which a chemical produces increasing responses. Irreversible antagonists bind to the receptor in an irreversible fashion and due to the high affinity for the receptor preclude binding of the agonist to the receptor. E. Correct! Competitive antagonists shift dose response curves to the right, decrease potency and increase the EC50. Competitive antagonists lead to a higher concentration of the agonist being required to overcome the effects of the antagonist. When it does overcome the antagonists, the maximal effect is the same as with the agonist alone. (E) Competitive antagonists

2 Question No. 2 of A 35 year old woman with a history of psychiatric illness exhibits newly developed urinary retention due to a neuroleptic. What is the recommended course of treatment? Question #02 (A) Carbachol (B) Bethanechol (C) Methacholine (D) Pilocarpine (E) Physostigmine Carbachol is applied ocularly. Bethanechol is used for postoperative and neurogenic ileus and urinary retention due to its ability to activate bowel and bladder smooth muscle. Methacholine is an inhaled muscarinic agonist that acts on airway smooth muscle, and is used as a challenge test for asthma. Pilocarpine stimulates sweat, tears and saliva. Physostigmine is used for glaucoma and atropine overdose. Bethanechol is used for postoperative and neurogenic ileus and urinary retention due to its ability to activate bowel and bladder smooth muscle. Bethanechol (B)Bethanechol

3 Question No. 3 of A 19 year old male requires a bactericidal antibiotic. Which of the following would not be appropriate in this situation? Question #03 (A) Vancomycin (B) Erythromycin (C) Penicillin (D) Cephalosporin antibiotics (E) Aminoglycoside antibiotics Think of a drug that is bacteriostatic, not bactericidal. Erythromycin is bacteriostatic, not bactericidal. Think of a drug that is bacteriostatic, not bactericidal. Think of a drug that is bacteriostatic, not bactericidal. Think of a drug that is bacteriostatic, not bactericidal. Erythromycin is bacteriostatic, not bactericidal. Erythromycin (B)Erythromycin

4 Question No. 4 of You are comparing the safety of two drugs that each treats the same disease. To compare the safety of the two drugs, you need to know each drugs : Question #04 (A) Drug Shape (B) Henderson-Hasselbalch Equation (C) K m (D) Therapeutic Index (E) Half-life Drug shape does not determine how safe a drug is. B. Incorrect! The Henderson-Hasselbalch Equation calculates the percentage of ionized & unionized molecules. K m indicates the affinity of the enzyme for the substrate in question. D. Correct! The therapeutic index indications the relative measure of the safety of a given drug. Half-life is the period of time that is required for the concentration or amount of drug in the body to be reduced to exactly one-half of a given concentration or amount. The therapeutic index gives a relative measure of how safe a drug is. TI equals the LD 50 (dose toxic to 50% of the population) divided by the ED 50 (dose effective in 50% of the population). This can be memorized using the abbreviation TILE, for TI = LD 50 /ED 50. Safer drugs have higher TI values. (D)Therapeutic Index

5 Question No. 5 of A 24 year old farm worker presents with miosis, diarrhea, cramping and blurry vision. You suspect possible exposure to: Question #05 (A) Parathion (B) Lead (C) Mercury (D) Iron (E) St. John s Wort A. Correct! These symptoms would be expected with parathion poisoning. B. Incorrect! The primary symptom of lead poisoning is neurotoxicity. The primary symptom of mercury poisoning is neurotoxicity. Think of a toxicant associated with agriculture. Think of a toxicant associated with agriculture. Parathion is an organophosphate, and organophosphates are commonly used in agriculture as pesticides. Remember the mnemonic LESS DUMBB to memorize the symptoms of cholinesterase inhibitor poisoning: Lacrimation Excitation of skeletal muscle and CNS Sweating Salivation Diarrhea Urination Miosis Bronchospasm, and Bradycardia (A)Parathion

6 Question No. 6 of 10 6., occurs when the rate of elimination is proportional to the drug concentration, such that a constant fraction of a drug is eliminated per unit time. Question #06 (A) Chemical antagonism (B) First-Order Elimination (C) Pharmacokinetic antagonism (D) Zero-order elimination (E) Bioavailability Chemical antagonism is the interaction of two substances in solution so that the effect of the active drug is lost. First-order kinetics occurs when the rate of elimination is proportional to the drug concentration. Pharmacokinetic antagonism is one drug affecting the absorption, metabolism or excretion of another drug. Zero-order elimination refers to elimination that remains at a constant rate irrespective of the amount of drug in the body. Bioavailability refers to the extent and rate at which the drug accesses the site of action. The C p (target plasma concentration) decreases exponentially over time, such that the graph of time versus plasma concentration is a curve. The rate of elimination is proportional to the drug concentration, such that a constant fraction of a drug (such as 20% of the amount in the body) is eliminated per a given unit of time. (B)First-Order Elimination

7 Question No. 7 of You are asked to participate in a Phase III clinical trial for a new drug called coritidine. What type of drug is coritidine, based on its nomenclature? Question #07 (A) Tricyclic antidepressant (B) H 2 antagonist (C) Local anesthetic (D) Barbiturate (E) Protease inhibitor TCAs tend to end in ipramine or triptyline. H 2 antagonists have the suffix tidine. Local anesthetics have the suffix caine. Barbiturates have the suffix barbital. Protease inhibitors have the suffix navir. H 2 antagonists have the suffix tidine. An example is cimetidine. (B)H 2 antagonist

8 Question No. 8 of You administer either Drug A or Drug A in combination with Drug B to a group of patients. You observe a dose-response curve as show below. What type of drug is Drug B? Question #08 (A) Antagonist (B) Non-competitive antagonist (C) Agonist (D) Irreversible antagonist (E) Competitive antagonist The drug is an antagonist, but there is a more specific answer available. Non-competitive antagonists decrease efficacy of the agonist and shift the doseresponse curves downward. Agonists initiate changes in cell function via acting at receptors. They have a specified potency, which is the range of doses over which a chemical produces increasing responses. Irreversible antagonists bind to the receptor in an irreversible fashion and due to the high affinity for the receptor preclude binding of the agonist to the receptor. Competitive antagonists shift dose response curves to the right, decrease potency and increase the EC50. Drug B is a non-competitive antagonist, which decreases efficacy of the agonist (Drug A) and shifts the curve downward. If you can imagine what is happening, as antagonists occupy the receptors and refuse to move, antagonists have fewer receptors they can occupy, and thus they cannot cause as great of an effect. (B) Non-competitive antagonist

9 Question No. 9 of A 67 year old woman exhibits signs of closed angle glaucoma. Which of the following would be an appropriate treatment for this condition? Question #09 (A) Neostigmine (B) Edrophonium (C) Pyridostigmine (D) Bethanechol (E) Physostigmine Neostigmine is used for postoperative and neurogenic ileus and urinary retention due to its ability to activate bowel and bladder smooth muscle. B. Incorrect! Edrophonium is a very short acting drug used to treat myasthenia gravis. Pyridostigmine is a long-acting drug used to treat myasthenia gravis because it does not cross the blood brain barrier. Bethanechol is used for postoperative and neurogenic ileus and urinary retention due to its ability to activate bowel and bladder smooth muscle. E. Correct! Nicotinic receptors are ligand-gated, Na + /K + channels. The indirect cholinergic agonists work primarily through the inhibition of acetylcholinesterase. The end result of all indirect agonists is an increase in endogenous levels of ACh, as the body is unable to break down the neurotransmitter once it is released into the synapse. The primary differences in these drugs are in their formulation and their ability to enter the body and cross the blood-brain barrier. Physostigmine is used in the eyes to treat glaucoma, which can be remembered using PHY is for the EYES. It is also useful for treating atropine overdose because it crosses the blood-brain barrier. Physostigmine (E)Physostigmine

10 Question No. 10 of The two lines on this graph represent the kinetics of two different drugs. What information correctly describes the graph? Question #10 (A) The drugs are not competing. (B) The drugs are competing for the same enzyme. (C) Drug A has a higher Vmax than Drug B. (D) Drug B has a lower Vmax than Drug A. (E) Neither drug is causing inhibition of the enzyme. Look at how the lines are crossing each other. In this graph, the drugs are competing for the same enzyme. Look at how the lines are crossing each other. Look at how the lines are crossing each other. Look at how the lines are crossing each other. If the lines of two different drugs cross each other, these two drugs are competing for the same enzyme. Thus, the answer is B, the drugs are competing for the same enzyme. The higher a drug crosses the y-intercept, the lower it Vmax, which is why answers C and D are incorrect. (B)The drugs are competing for the same enzyme.