10 USMLE Toxicology and Pharmacology Questions with Answers and Explanations (Set 1)

These 10 USMLE toxicology and pharmacology practice questions cover poisonings and antidotes, acid-base changes in aspirin overdose, anticoagulant mechanisms, drug-induced lupus, drugs to avoid in the elderly, occupational nitrate exposure and drugs to avoid in pregnancy. Each question includes the correct answer and an explanation.

Written by Dr. Ray Makar, MD, author of the Repeated USMLE Questions eBook series.

Prefer video? Watch daily USMLE case questions on MedFocus by Dr. Ray on YouTube.

How to use this page: read the question, pick your answer, then tap Show Answer and Explanation.

Questions

Question 1. A 35-year-old man is brought to the ER by his coworker complaining of shortness of breath and dizziness. He was acting hysterical most of the day before he felt dizzy and needed help. They have worked at a plastics manufacturing plant for seven years. What is the treatment needed to be given instantly?

A- Ethyl alcohol
B- Sodium nitrite
C- Activated charcoal
D- Atropine
E- Flumazenil

Based on the symptoms and the work environment described, the most likely diagnosis is cyanide poisoning, which can occur in industrial settings like plastic manufacturing.
Sodium nitrite is part of the treatment for cyanide poisoning as it helps to convert hemoglobin into methemoglobin, which binds cyanide, allowing it to be removed from the body.
The correct answer is B
Note: hydroxocobalamin is now the preferred cyanide antidote where it is available.

 

Question 2. A 64-year-old man complains of severe headache, dizziness, nausea and deterioration of vision. Workup shows a decrease in oxygen carrying capacity with no change in PO2 of arterial blood. Which of the following is most likely responsible for this patient’s symptoms?

A- Ozone
B- Carbon monoxide (CO)
C- Sulfur dioxide
D- Nitrogen

This is a typical presentation of carbon monoxide poisoning. It is characterized by decreased oxygen capacity with no change in PO2 of arterial blood. Elevated level of carboxyhemoglobin should confirm the diagnosis. CO poisoning could cause irreversible brain damage or even death if it happens during sleep. Carbon monoxide is a colorless, odorless and tasteless gas which could be accumulated in the body without noticing it until too late when symptoms occur.
Ozone toxicity leads to chest pain, cough, shortness of breath and sore throat. Sulfur dioxide toxicity is causing cough, runny nose and difficulty breathing due to swollen lungs. Nitrogen toxicity is very rare and is known as inert gas asphyxiation which is used as a suicide method.
The correct answer is B

 

Question 3. A 12-year-old girl visits her physician for follow up after treatment with methotrexate for acute lymphoblastic leukemia. Stool analysis reveals blood in stool. Her platelet count is very low. Which of these substances should be administered immediately?

A- Vitamin K
B- Leucovorin
C- Deferoxamine
D- Amitriptyline
E- Penicillamine

This is a case of methotrexate toxicity. Leucovorin is the active form of folic acid and it reverses methotrexate toxicity except neurotoxicity. Vitamin K is used in cases of bleeding due to defect in coagulation factors. Deferoxamine is the drug used in iron overdose. Amitriptyline is a tricyclic antidepressant and it has no role in methotrexate toxicity. Penicillamine is used in Wilson’s disease and some other metal toxicities.
The correct answer is B

 

Question 4. A 62-year-old man who has a history of various drug overdose incidents presents to the emergency room after administration of a high dose of aspirin. Which set of these findings characterizes this condition?

A- Metabolic acidosis and respiratory acidosis
B- Metabolic acidosis and respiratory alkalosis
C- Metabolic alkalosis and respiratory acidosis
D- Metabolic alkalosis and respiratory alkalosis

Aspirin (salicylate) overdose causes a mixed acid-base disorder. Early on, salicylate directly stimulates the respiratory center, causing hyperventilation and respiratory alkalosis. Then it uncouples oxidative phosphorylation, so lactic acid and ketoacids build up and cause an anion-gap metabolic acidosis. Treatment includes activated charcoal, IV sodium bicarbonate to alkalinize the urine, and hemodialysis in severe cases.
The correct answer is B

 

Question 5. A 64-year-old man arrives at the emergency room after recurrent vomiting of blood for the last 2 hours. He recently underwent right hip replacement and had taken heparin to prevent thromboembolic complications. Internal bleeding is suspected and treatment with protamine has begun. What kind of antagonism is related to this treatment?

A- Chemical
B- Competitive
C- Irreversible
D- Physiological

Protamine antagonizes the action of heparin by binding to heparin itself preventing its action therefore, it is considered chemical antagonism. Competitive inhibition is the binding of an inhibitor to the site of action of another drug preventing it from acting on these sites i.e. receptors. Irreversible inhibition is the binding of the inhibitor to the active substance i.e. enzymes and eventually blocking the active substrates to bind to it. Action of insulin on certain receptor-effector system to decrease blood glucose is considered physiological inhibition.
The correct answer is A

 

Question 6. A 63-year-old man with a history of atrial fibrillation visits the clinic for evaluation of his current medications. His physician orders laboratory tests including prothrombin time. Which one of these drugs is used to reduce the possibility of blood clotting in this patient through its action on the enzyme vitamin K epoxide reductase?

A- Aspirin
B- Dipyridamole
C- Heparin
D- Streptokinase
E- Warfarin

This patient takes warfarin to reduce the possibility of blood clotting which is a common complication of atrial fibrillation. Warfarin acts by inhibiting the enzyme vitamin K epoxide reductase which is responsible for converting vitamin K into active form. Most of the clotting factors are dependent on vitamin K to participate in thrombosis. Aspirin and dipyridamole act by inhibition of platelet aggregation. Heparin is used in emergencies by increasing the activity of antithrombin III which inhibits thrombin and other coagulation factors. Streptokinase acts by converting plasminogen into plasmin which results in digesting fibrin clots.
The correct answer is E

 

Question 7. A 54-year-old woman with a history of hypothyroidism, hypertension, depression and joint pains develops fatigue, facial rash, headaches, dizziness, confusion, diarrhea and personality changes. She takes levothyroxine, hydralazine, amitriptyline and ibuprofen. Which of these drugs is most likely responsible for this patient’s new symptoms?

A- Levothyroxine
B- Hydralazine
C- Amitriptyline
D- Ibuprofen

This patient has lupus-like symptoms. Hydralazine is one of the drugs that cause lupus-like changes. Other drugs are isoniazid, chlorpromazine, penicillamine, procainamide, interferons and quinidine.
The correct answer is B

 

Question 8. A 78-year-old woman presents to the office with her daughter. Last week, she fell twice while she was walking with her daughter. She was not shaking or lost her consciousness and she got up in a minute. Her medical history is significant for hypertension and osteoarthritis. She takes Lisinopril daily and ibuprofen as needed. She also takes diphenhydramine to help her sleeping during the night. Her vital signs are within normal limits on sitting and standing. All her laboratory tests are within normal ranges. Her mini-mental status test is normal. What is the best explanation for this patient’s fallings?

A- Alzheimer’s disease
B- Old age
C- Osteoarthritis
D- Postural hypotension
E- Side effect of one of her medications

One of the concerning side effects of diphenhydramine is dizziness and disturbed coordination which leads to multiple falls. Other choices are excluded by history, examination and normal laboratory tests. Old age does not result in falls unless there is accompanied illness.
The correct answer is E

 

Question 9. A 40-year-old man who works in an explosives factory presents to the clinic for a routine physical examination. He reports episodes of headache and palpitations every Monday. Physical examination, laboratory tests and ECG are normal. He asks whether the chemicals at his job could be causing his symptoms. What is the most appropriate response?

A- No, because the symptoms happen on Monday after a weekend away from work
B- No, because the symptoms are episodic
C- Yes, they are caused by re-exposure to nitroglycerin after his tolerance is lost over the weekend
D- Yes, they are caused by an allergy to the chemicals

This is “Monday disease” from occupational nitroglycerin exposure. During the work week, workers develop tolerance to the vasodilating effect of nitroglycerin. Over the weekend the tolerance is lost, so re-exposure on Monday causes vasodilation with headache, flushing and reflex tachycardia (palpitations). Being away from nitrates over the weekend can also cause rebound coronary vasoconstriction, with angina or even myocardial infarction.
The correct answer is C

 

Question 10. A 9-month-old girl is brought by her grandmother to the clinic for routine examination. She told you that the girl is totally fine but she noticed yellow discoloration of her teeth. The physician asks if her mother took any medications during her pregnancy. She remembers that she was taking an antibiotic for bronchitis. Which of these antibiotics is the most likely responsible for this condition?

A- Chloramphenicol
B- Cephalexin
C- Tetracycline
D- Ampicillin
E- Erythromycin

Tetracycline is the most likely antibiotic responsible for the yellow discoloration of teeth in infants when mothers take it during pregnancy. Tetracycline use is uncommon in the USA and is contraindicated in pregnancy and early childhood.
The correct answer is C

 

High-yield points from this set

  • Cyanide poisoning (industrial fires, plastics, nitroprusside): hydroxocobalamin, or nitrites plus thiosulfate.
  • CO poisoning lowers oxygen-carrying capacity with a normal arterial PO2; give 100% oxygen.
  • Methotrexate toxicity is reversed with leucovorin (folinic acid).
  • Aspirin overdose = respiratory alkalosis plus anion-gap metabolic acidosis.
  • Protamine binds heparin directly: chemical antagonism.
  • Warfarin inhibits vitamin K epoxide reductase; monitor with PT/INR.
  • Drug-induced lupus: hydralazine, procainamide, isoniazid (anti-histone antibodies).
  • Diphenhydramine (anticholinergic) causes falls and confusion in the elderly.
  • Headache every Monday in explosives workers = nitroglycerin re-exposure (“Monday disease”).
  • Tetracyclines discolor teeth; avoid in pregnancy and young children.

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