Antimicrobials — MCQs

A patient on digoxin therapy presents with atrial fibrillation and controlled ventricular rate. Upon evaluation, the patient's serum digoxin levels are elevated compared to previous values. Which of the following concomitant medications is most likely to have contributed to the enhanced digoxin toxicity?

Q16

Which antitubercular drug reduces the efficacy of oral contraceptive pills (OCPs)?

Q17

What is the mechanism of action of Methotrexate?

Q18

What is the mechanism of action of Tocilizumab?

Q19

After receiving a positive newborn screening result, a 2-week-old male infant is brought to the pediatrician for a diagnostic sweat test. The results demonstrated chloride levels of 65 mmol/L (nl < 29 mmol/L). Subsequent DNA sequencing revealed a 3 base pair deletion in a transmembrane cAMP-activated ion channel known to result in protein instability and early degradation. The physician discusses with the parents that the infant will develop respiratory infections due to improper mucus clearance and reviews various mucolytic agents, such as one that cleaves disulfide bonds between mucus glycoproteins thereby loosening the mucus plug. This mucolytic can also be used as a treatment for which of the following overdoses?

Q20

A 36-year-old woman with no significant medical history presents with a four-week history of epigastric pain. The pain tends to occur two hours after meals. She has lost 4 pounds over the last four weeks. She is allergic to azithromycin and clarithromycin. A urea breath test detects radiolabeled carbon dioxide in exhaled breath. Two days after starting definitive treatment, she returns to the hospital with flushing, headaches, nausea and vomiting after having a few beers that night. What is the mechanism of the drug involved in the adverse reaction?

Antimicrobials US Medical PG Practice Questions and MCQs

Question 11: Which of the following are toxic metabolites associated with methanol poisoning?

A. Formic acid + Oxalic acid
B. Formic acid + Lactic acid (Correct Answer)
C. Glycolic acid + Oxalic acid
D. Oxalic acid + Lactic acid
E. Hippuric acid + Formic acid

Explanation: ***Formic acid + Lactic acid*** - **Methanol** is metabolized in the body into **formaldehyde** and then to **formic acid**, which is primarily responsible for its toxicity. - While not a direct derivative, **lactic acid** levels often rise significantly in severe methanol poisoning due to **hypoxia** and **metabolic acidosis** caused by formic acid-induced mitochondrial dysfunction. *Formic acid + Oxalic acid* - **Formic acid** is indeed a toxic metabolite of methanol, but **oxalic acid** is not. - **Oxalic acid** is a toxic metabolite of **ethylene glycol** poisoning, not methanol. *Glycolic acid + Oxalic acid* - Neither **glycolic acid** nor **oxalic acid** are toxic metabolites of methanol. - Both **glycolic acid** and **oxalic acid** are associated with **ethylene glycol** poisoning. *Oxalic acid + Lactic acid* - **Oxalic acid** is not a derivative of methanol poisoning. - While **lactic acid** can be elevated in methanol poisoning, its combination with oxalic acid is incorrect for methanol metabolites. *Hippuric acid + Formic acid* - **Formic acid** is correct, but **hippuric acid** is not a metabolite of methanol. - **Hippuric acid** is a metabolite of **toluene** and is used as a urinary marker for toluene exposure.

Question 12: Phenobarbitone exerts its therapeutic effect primarily through which mechanism?

A. Enhancement of GABA-A receptor function (Correct Answer)
B. Voltage-gated sodium channel blockade
C. Voltage-gated calcium channel blockade
D. Potassium channel opening
E. NMDA receptor antagonism

Explanation: ***Enhancement of GABA-A receptor function*** - **Phenobarbitone** (phenobarbital) is a barbiturate that acts primarily by **enhancing GABA-A receptor activity** - It binds to a distinct **barbiturate binding site** on the GABA-A receptor complex - This binding **prolongs the duration of chloride channel opening** in response to GABA - Results in increased **chloride ion influx**, neuronal **hyperpolarization**, and **decreased neuronal excitability** - This mechanism underlies its **anticonvulsant** and **sedative-hypnotic** properties *NMDA receptor antagonism* - While some anticonvulsants work through NMDA receptor antagonism (e.g., ketamine, felbamate), this is **not the primary mechanism** of phenobarbitone *Voltage-gated sodium channel blockade* - Sodium channel blockade is the mechanism of action for drugs like **phenytoin, carbamazepine**, and **lamotrigine** - Phenobarbitone does not significantly block sodium channels at therapeutic concentrations *Voltage-gated calcium channel blockade* - Calcium channel blockers like **ethosuximide** (T-type) and **gabapentin** (α2δ subunit) work through this mechanism - This is not the primary mechanism for phenobarbitone *Potassium channel opening* - Potassium channel openers like **retigabine** increase potassium conductance - Phenobarbitone does not primarily work through this mechanism

Question 13: A patient on multidrug therapy (MBMDT) for leprosy presents with inflammation over preexisting lesions and nerve involvement. What is the best approach for treatment?

A. Continue anti-leprosy treatment (ALT) and start thalidomide
B. Continue anti-leprosy treatment (ALT) and start steroids (Correct Answer)
C. Stop anti-leprosy treatment (ALT) and start thalidomide
D. Stop anti-leprosy treatment (ALT) and start steroids
E. Stop anti-leprosy treatment (ALT) temporarily and reassess

Explanation: ***Continue anti-leprosy treatment (ALT) and start steroids*** - The patient is experiencing a **type 1 leprosy reaction (reversal reaction)**, characterized by inflammation over existing lesions and nerve involvement, which requires immediate immunosuppression with **steroids**. - **Anti-leprosy treatment (ALT)** must be continued as the reaction is an immunological phenomenon independent of the ongoing bacterial eradication and stopping it could lead to relapse. *Continue anti-leprosy treatment (ALT) and start thalidomide* - **Thalidomide** is primarily used for **type 2 leprosy reactions (erythema nodosum leprosum, ENL)**, which present with new, painful, tender nodules, fever, and systemic symptoms, not typically aggravated pre-existing lesions and nerve involvement. - While ALT is correctly continued, thalidomide is not the first-line treatment for a type 1 reaction due to its specific indication for ENL. *Stop anti-leprosy treatment (ALT) and start thalidomide* - **Stopping ALT** is incorrect as it can lead to **relapse** of leprosy and development of **drug resistance**. - **Thalidomide** is not indicated for type 1 leprosy reactions, making this approach inappropriate. *Stop anti-leprosy treatment (ALT) and start steroids* - While **steroids** are the correct treatment for **type 1 leprosy reactions**, **stopping ALT** is a critical error that can have severe consequences for the patient's leprosy management. - The reaction is an immune response to dead or dying bacilli, not a failure of ALT, so treatment should be completed. *Stop anti-leprosy treatment (ALT) temporarily and reassess* - **Temporarily stopping ALT** is incorrect even during a leprosy reaction, as reactions are immune-mediated responses to treatment, not adverse drug effects requiring cessation. - Interrupting treatment can lead to **incomplete bacterial eradication**, **relapse**, and potential **drug resistance**; the standard approach is to continue ALT while managing the reaction with immunosuppression.

Question 14: A patient is taking metronidazole for anaerobic infection. Which of the following should be avoided during this period

A. Colchicine
B. Alcohol (Correct Answer)
C. Rifampicin
D. Ciprofloxacin
E. Warfarin

Explanation: ***Alcohol*** - Metronidazole inhibits **aldehyde dehydrogenase**, an enzyme responsible for metabolizing alcohol, leading to an accumulation of **acetaldehyde**. - This accumulation causes a **disulfiram-like reaction**, characterized by flushing, nausea, vomiting, headache, and palpitations, making alcohol avoidance crucial. *Colchicine* - Colchicine is used for gout and **familial Mediterranean fever**; there's no major contraindicated interaction with metronidazole. - While both can cause gastrointestinal side effects, combining them does not typically lead to a life-threatening interaction or necessitate avoidance. *Rifampicin* - Rifampicin is a potent **CYP450 enzyme inducer** and can decrease the effectiveness of many drugs, including metronidazole, by increasing its metabolism. - While an interaction exists, it typically involves reduced metronidazole efficacy rather than a strict contraindication requiring complete avoidance. *Ciprofloxacin* - Ciprofloxacin is a **quinolone antibiotic** and generally does not have a clinically significant, contraindicated interaction with metronidazole. - Both can individually cause **gastrointestinal side effects**, but their co-administration is not generally discouraged due to synergy in treatment or increased toxicity. *Warfarin* - Metronidazole can enhance the anticoagulant effect of warfarin by inhibiting its metabolism, potentially increasing **INR** and bleeding risk. - However, this interaction is **manageable with monitoring**; warfarin is not absolutely contraindicated and can be used with dose adjustments and frequent INR checks, unlike alcohol which must be completely avoided.

Question 15: A patient on digoxin therapy presents with atrial fibrillation and controlled ventricular rate. Upon evaluation, the patient's serum digoxin levels are elevated compared to previous values. Which of the following concomitant medications is most likely to have contributed to the enhanced digoxin toxicity?

A. Triamterene
B. KCL
C. Atenolol
D. Clarithromycin (Correct Answer)
E. Amiodarone

Explanation: ***Clarithromycin*** - **Clarithromycin** is a **macrolide antibiotic** known to inhibit the cytochrome P450 3A4 (CYP3A4) enzyme system and **P-glycoprotein**. - This inhibition leads to decreased metabolism and **efflux of digoxin**, resulting in **increased serum digoxin levels** and enhanced toxicity. - Among the options, clarithromycin is the **most common cause** of elevated digoxin levels through P-gp inhibition. *Triamterene* - **Triamterene** is a **potassium-sparing diuretic** that can increase serum potassium. - **Hyperkalemia** generally *reduces* the binding of digoxin to Na+/K+-ATPase, thereby potentially *reducing* its toxic effects. - Does not significantly affect digoxin serum levels. *KCL* - **Potassium chloride (KCl)** is used to correct **hypokalemia**. - **Hypokalemia** can *potentiate* digoxin toxicity because low potassium increases digoxin binding to the Na+/K+-ATPase pump. - However, KCl supplementation *corrects* hypokalemia and would actually *reduce* toxicity risk, not increase serum digoxin levels. *Atenolol* - **Atenolol** is a **beta-blocker** primarily used to control heart rate and blood pressure. - While it can slow the heart rate like digoxin (additive pharmacodynamic effect), it does not significantly alter the **pharmacokinetics** or serum levels of digoxin. *Amiodarone* - **Amiodarone** is an **antiarrhythmic** that can inhibit P-glycoprotein and increase digoxin levels. - However, in this scenario, **clarithromycin** is more commonly associated with acute elevations in digoxin levels in clinical practice. - Amiodarone interactions are well-known and typically require dose adjustments at initiation.

Question 16: Which antitubercular drug reduces the efficacy of oral contraceptive pills (OCPs)?

A. Rifampicin (Correct Answer)
B. Isoniazid
C. Ethambutol
D. Pyrazinamide
E. Streptomycin

Explanation: ***Rifampicin*** - **Rifampicin** is a potent inducer of **cytochrome P450 enzymes**, particularly CYP3A4, which metabolize oral contraceptive pills (OCPs). - This increased metabolism leads to lower systemic levels of contraceptive hormones, reducing their efficacy and increasing the risk of **unintended pregnancy**. - Women on Rifampicin should use **additional barrier contraception** or alternative contraceptive methods. *Isoniazid* - **Isoniazid** is primarily metabolized by N-acetyltransferase and cytochrome P450, but it is not a significant enzyme inducer. - It does not typically interfere with the effectiveness of **oral contraceptive pills**. *Ethambutol* - **Ethambutol** is eliminated largely unchanged via renal excretion and is not a significant inducer or inhibitor of cytochrome P450 enzymes. - It does not interact with **oral contraceptive pills**. *Pyrazinamide* - **Pyrazinamide** is metabolized by the liver, but it does not significantly induce or inhibit the cytochrome P450 system involved in OCP metabolism. - It is not known to reduce the effectiveness of **oral contraceptive pills**. *Streptomycin* - **Streptomycin** is an aminoglycoside antibiotic that is not metabolized by the liver and does not affect cytochrome P450 enzymes. - It has no interaction with **oral contraceptive pills**.

Question 17: What is the mechanism of action of Methotrexate?

A. Inhibition of Dihydrofolate reductase (Correct Answer)
B. Inhibits pyrimidine synthesis
C. Inhibits cell replication by acting on G phase of cell cycle
D. Inhibits Thymidylate synthase
E. Inhibits RNA polymerase

Explanation: ***Inhibition of Dihydrofolate reductase*** - **Methotrexate** is a **folate analog** that competitively inhibits **dihydrofolate reductase (DHFR)**, an enzyme essential for converting **dihydrofolate** to **tetrahydrofolate**. - This inhibition blocks the synthesis of **purines** and **pyrimidines**, thereby preventing DNA and RNA synthesis and ultimately inhibiting cell proliferation. *Inhibits pyrimidine synthesis* - While methotrexate ultimately inhibits pyrimidine synthesis by depleting tetrahydrofolate, its direct mechanism is not the inhibition of the pyrimidine synthesis pathway enzymes themselves. - Its primary action is upstream, by inhibiting DHFR. *Inhibits cell replication by acting on G phase of cell cycle* - Methotrexate primarily inhibits cells in the **S-phase** of the cell cycle, as it interferes with DNA synthesis. - It does not specifically target the G phase; rather, it affects cells that are actively attempting to replicate their DNA. *Inhibits Thymidylate synthase* - **Thymidylate synthase** is inhibited by drugs like **5-fluorouracil**, which directly blocks the conversion of **deoxyuridine monophosphate (dUMP)** to **deoxythymidine monophosphate (dTMP)**. - Methotrexate's effect on thymidylate synthesis is indirect, mediated by the depletion of the cofactor **N5,N10-methylene-tetrahydrofolate** due to DHFR inhibition. *Inhibits RNA polymerase* - **RNA polymerase** inhibition is the mechanism of drugs like **rifampin** (bacterial RNA polymerase) and **α-amanitin** (eukaryotic RNA polymerase). - Methotrexate does not directly inhibit RNA polymerase; its effects on RNA synthesis are secondary to depletion of nucleotide precursors through DHFR inhibition.

Question 18: What is the mechanism of action of Tocilizumab?

A. TNF Alpha inhibition
B. IL-18 inhibition
C. Inhibits binding of IL-6 to its receptor IL-6R (Correct Answer)
D. Inhibits binding of IL-1
E. IL-17 inhibition

Explanation: ***Inhibits binding of IL-6 to its receptor IL-6R*** - **Tocilizumab** is a **monoclonal antibody** that specifically targets the **interleukin-6 (IL-6) receptor**. - By blocking IL-6 from binding to its receptor, Tocilizumab **inhibits IL-6 mediated signaling**, thereby reducing inflammation and immune responses. *TNF Alpha inhibition* - **TNF-alpha inhibitors** (e.g., adalimumab, infliximab) target **tumor necrosis factor-alpha**, a different pro-inflammatory cytokine. - While both TNF-alpha and IL-6 are involved in inflammatory diseases, their signaling pathways and therapeutic targets are distinct. *IL-18 inhibition* - **IL-18** is another pro-inflammatory cytokine, but Tocilizumab does **not directly target** or inhibit its activity. - Drugs that target IL-18 or its pathways are distinct from those targeting IL-6. *IL-17 inhibition* - **IL-17 inhibitors** (e.g., secukinumab, ixekizumab) are biologics that target **interleukin-17**, particularly useful in psoriasis and ankylosing spondylitis. - Tocilizumab's mechanism is specific to **IL-6**, not IL-17. *Inhibits binding of IL-1* - **Interleukin-1 (IL-1)** is a key mediator of inflammation, but specific **IL-1 inhibitors** (e.g., anakinra, canakinumab) act by blocking IL-1 or its receptor. - Tocilizumab's mechanism is specific to the **IL-6 cytokine-receptor interaction**, not IL-1.

Question 19: After receiving a positive newborn screening result, a 2-week-old male infant is brought to the pediatrician for a diagnostic sweat test. The results demonstrated chloride levels of 65 mmol/L (nl < 29 mmol/L). Subsequent DNA sequencing revealed a 3 base pair deletion in a transmembrane cAMP-activated ion channel known to result in protein instability and early degradation. The physician discusses with the parents that the infant will develop respiratory infections due to improper mucus clearance and reviews various mucolytic agents, such as one that cleaves disulfide bonds between mucus glycoproteins thereby loosening the mucus plug. This mucolytic can also be used as a treatment for which of the following overdoses?

A. Cyanide
B. Acetaminophen (Correct Answer)
C. Salicylates
D. Benzodiazepines
E. Opioids

Explanation: ***Acetaminophen*** - The mucolytic described is **N-acetylcysteine (NAC)**, which functions by cleaving **disulfide bonds** in mucus glycoproteins. - NAC is also the antidote for **acetaminophen overdose**, as it replenishes **glutathione stores**, which are crucial for detoxifying acetaminophen metabolites. *Cyanide* - Cyanide poisoning is typically treated with agents like **amyl nitrite**, **sodium nitrite**, and **sodium thiosulfate**. - These treatments work by inducing methemoglobinemia or directly binding to cyanide to facilitate its excretion. *Salicylates* - Salicylate (aspirin) overdose is primarily managed with **alkalinization of the urine** (using sodium bicarbonate) and **hemodialysis**. - These methods enhance salicylate elimination. *Benzodiazepines* - Benzodiazepine overdose is treated with **flumazenil**, a **GABA receptor antagonist**, which reverses the central nervous system depression. - NAC has no role in reversing the effects of benzodiazepines. *Opioids* - Opioid overdose is treated with **naloxone**, an **opioid receptor antagonist**, which rapidly reverses respiratory depression and other opioid effects. - NAC does not interact with the opioid system.

Question 20: A 36-year-old woman with no significant medical history presents with a four-week history of epigastric pain. The pain tends to occur two hours after meals. She has lost 4 pounds over the last four weeks. She is allergic to azithromycin and clarithromycin. A urea breath test detects radiolabeled carbon dioxide in exhaled breath. Two days after starting definitive treatment, she returns to the hospital with flushing, headaches, nausea and vomiting after having a few beers that night. What is the mechanism of the drug involved in the adverse reaction?

A. Inhibition of aldehyde dehydrogenase (Correct Answer)
B. Coating of the gastric lining
C. Inhibition of H+/K+ ATPase in parietal cells
D. Binding to the 30S subunit of the ribosome
E. Binding to the 50S subunit of the ribosome

Explanation: ***Inhibition of aldehyde dehydrogenase*** - The patient's symptoms (flushing, headaches, nausea, vomiting) after consuming alcohol are characteristic of a **disulfiram-like reaction**. This occurs when a drug inhibits **aldehyde dehydrogenase**, leading to an accumulation of acetaldehyde, a toxic metabolite of alcohol. - Given the patient's positive urea breath test, she was likely diagnosed with *Helicobacter pylori* infection and was undergoing eradication therapy. In this scenario, **metronidazole** is often used in regimens, especially when macrolide allergies (azithromycin, clarithromycin) are present, and it is known to cause a disulfiram-like reaction by inhibiting aldehyde dehydrogenase. *Coating of the gastric lining* - This is the mechanism of action of **sucralfate**, a drug used to treat peptic ulcers by forming a protective barrier over the ulcer surface. - Sucralfate does not interact with alcohol to cause systemic adverse reactions like flushing or nausea and vomiting. *Inhibition of H+/K+ ATPase in parietal cells* - This is the mechanism of action of **proton pump inhibitors (PPIs)** like omeprazole or pantoprazole, which are commonly used in *H. pylori* eradication regimens to reduce gastric acid production. - PPIs do not cause a disulfiram-like reaction with alcohol. *Binding to the 30S subunit of the ribosome* - This is the mechanism of action of **tetracycline antibiotics** (e.g., doxycycline), which are sometimes used in *H. pylori* eradication regimens. - While tetracyclines can have various side effects, they do not cause a disulfiram-like reaction when combined with alcohol. *Binding to the 50S subunit of the ribosome* - This is the mechanism of action for **macrolide antibiotics** (like azithromycin, clarithromycin) and **chloramphenicol**. The patient is allergic to macrolides, ruling them out for primary treatment. - Although chloramphenicol can cause a disulfiram-like reaction, its use in *H. pylori* infection is not standard practice, and macrolides do not typically cause this reaction.

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