Antimicrobial Agents — MCQs
On this page
What is the most serious side effect of amphotericin B?
The development of resistance to conventional treatment has led WHO to recommend the use of combination therapies containing aemisinin derivatives (aemisinin-based combination therapies also known as ACTs). All of the following combination therapies are recommended if such resistance is suspected, except?
What is the mechanism of action of cephalosporins?
Which among the following is not a beta-lactamase resistant Penicillin?
All of the following are anti-HIV agents except?
Which of the following penicillins has the best gram-negative spectrum?
Which of the following is NOT an antiviral drug?
Which drug is strictly contraindicated in a pregnant female?
Which of the following statements about vancomycin is NOT true?
A 65-year-old male with pneumonia has a sputum culture positive for a staphylococcal strain that is -lactamase-positive. Which is the best choice of penicillin therapy in this patient?
Antimicrobial Agents Indian Medical PG Practice Questions and MCQs
Question 1171: What is the most serious side effect of amphotericin B?
- A. Hepatic damage
- B. Renal damage (Correct Answer)
- C. Cardiotoxicity
- D. Hypochromic anemia
Explanation: **Explanation:** **Amphotericin B** is a potent polyene antifungal used for systemic mycoses. Its mechanism involves binding to ergosterol in the fungal cell membrane, creating pores that lead to cell death. **1. Why Renal Damage is the Correct Answer:** Nephrotoxicity is the **most serious and dose-limiting** side effect of Amphotericin B, occurring in nearly 80% of patients. It occurs via two mechanisms: * **Direct Toxicity:** It causes vasoconstriction of the afferent arterioles, leading to decreased renal blood flow and GFR. * **Tubular Damage:** It increases the permeability of the distal tubule, leading to wasting of electrolytes (hypokalemia and hypomagnesemia) and Renal Tubular Acidosis (Type 1). **2. Why Other Options are Incorrect:** * **A. Hepatic damage:** Unlike azoles, Amphotericin B is not significantly hepatotoxic. * **C. Cardiotoxicity:** While rapid infusion can cause arrhythmias (due to acute hyperkalemia), it is not the primary or most common serious side effect compared to renal failure. * **D. Hypochromic anemia:** Amphotericin B causes **normocytic normochromic anemia** due to decreased erythropoietin production by the damaged kidneys, but it is rarely the most "serious" acute concern. **3. NEET-PG High-Yield Pearls:** * **Liposomal Amphotericin B:** Developed to reduce nephrotoxicity by targeting the drug specifically to fungal cells and reticuloendothelial systems, sparing the kidneys. * **Pre-loading:** Administering a **normal saline bolus** (saline loading) before infusion helps mitigate renal damage. * **Infusion-related reactions:** Often called "Shake and Bake" (fever, chills, rigors). This is managed with antipyretics, antihistamines, or hydrocortisone.
Question 1172: The development of resistance to conventional treatment has led WHO to recommend the use of combination therapies containing aemisinin derivatives (aemisinin-based combination therapies also known as ACTs). All of the following combination therapies are recommended if such resistance is suspected, except?
- A. Aemisether plus lumefantrine
- B. Aesunate plus quinine (Correct Answer)
- C. Aesunate plus pyrimethamine-sulfadoxine
- D. Aesunate plus mefloquine
Explanation: The World Health Organization (WHO) recommends **Artemisinin-based Combination Therapies (ACTs)** as the first-line treatment for uncomplicated *P. falciparum* malaria to combat drug resistance [1], [2]. The core principle of ACT is to combine a rapid-acting artemisinin derivative (which reduces parasite biomass quickly) with a long-acting partner drug (which eliminates remaining parasites). **Why Option B is the Correct Answer:** **Artesunate plus Quinine** is **not** an ACT. Quinine is a short-acting alkaloid, not a long-acting partner drug. While artesunate and quinine are both used in the management of *severe* malaria (often sequentially), they are not recommended as a standard ACT combination for uncomplicated malaria. Quinine requires a thrice-daily dosing for 7 days, which leads to poor compliance compared to the simplified 3-day ACT regimens. **Analysis of Incorrect Options (Recommended ACTs):** * **Option A (Artemether + Lumefantrine):** The most widely used ACT globally (e.g., Coartem) [2], [3]. * **Option C (Artesunate + Sulfadoxine-Pyrimethamine):** Recommended in areas where SP efficacy remains high. * **Option D (Artesunate + Mefloquine):** Highly effective but avoided in patients with neuropsychiatric history [3]. **High-Yield NEET-PG Pearls:** 1. **First-line for Uncomplicated Malaria (India):** Artesunate + SP (except in North-Eastern states, where **Artemether + Lumefantrine** is used due to SP resistance). 2. **Severe Malaria:** IV Artesunate is the drug of choice (preferred over IV Quinine). 3. **Pregnancy:** ACTs are now recommended by WHO in the **first trimester** for uncomplicated falciparum malaria (previously only 2nd and 3rd). 4. **Mechanism:** Artemisinins act by releasing free radicals via the cleavage of their endoperoxide bridge in the presence of heme.
Question 1173: What is the mechanism of action of cephalosporins?
- A. Interferes with cell wall synthesis (Correct Answer)
- B. Inhibition of DNA gyrase
- C. Inhibition of protein synthesis
- D. Inhibition of DNA polymerase
Explanation: **Explanation:** **1. Why Option A is Correct:** Cephalosporins are **Beta-lactam antibiotics**. Their primary mechanism involves inhibiting the synthesis of the bacterial cell wall. They bind to and inhibit **Penicillin-Binding Proteins (PBPs)**, which are transpeptidase enzymes responsible for cross-linking peptidoglycan chains. This disruption leads to the formation of a weak cell wall, resulting in bacterial lysis (bactericidal action). **2. Why Other Options are Incorrect:** * **Option B (DNA Gyrase):** This is the mechanism of **Fluoroquinolones** (e.g., Ciprofloxacin). They inhibit DNA gyrase (Topoisomerase II) and Topoisomerase IV, preventing DNA supercoiling and replication. * **Option C (Protein Synthesis):** This is the mechanism for several classes, including **Aminoglycosides, Tetracyclines, Macrolides, and Chloramphenicol**, which target the 30S or 50S ribosomal subunits. * **Option D (DNA Polymerase):** This is primarily the mechanism of certain **Antiviral drugs** (e.g., Acyclovir) or the anti-cancer drug Cytarabine, rather than standard antibacterial agents. **3. NEET-PG High-Yield Clinical Pearls:** * **Resistance:** The most common mechanism of resistance against cephalosporins is the production of **Beta-lactamases** (ESBLs). * **Cross-Reactivity:** There is a **5-10% risk of cross-sensitivity** between penicillins and cephalosporins; avoid in patients with a history of anaphylaxis to penicillin. * **Disulfiram-like Reaction:** Cephalosporins containing a **methylthiotetrazole (MTT) side chain** (e.g., Cefotetan, Cefoperazone) can cause disulfiram-like reactions with alcohol and hypoprothrombinemia (bleeding risk). * **Generation Rule:** As we move from 1st to 4th generation, Gram-negative activity increases while Gram-positive activity generally decreases (except for the 4th and 5th generations).
Question 1174: Which among the following is not a beta-lactamase resistant Penicillin?
- A. Methicillin
- B. Carbenicillin (Correct Answer)
- C. Nafcillin
- D. Oxacillin
Explanation: **Explanation** The beta-lactamase resistant penicillins (also known as **Antistaphylococcal Penicillins**) are specifically designed to resist hydrolysis by staphylococcal penicillinase. This resistance is achieved through the addition of a bulky side chain that sterically hinders the beta-lactamase enzyme from reaching the beta-lactam ring. **Why Carbenicillin is the Correct Answer:** **Carbenicillin** belongs to the **Antipseudomonal Penicillins** (specifically the Carboxypenicillin group). While it has an extended spectrum against Gram-negative bacteria like *Pseudomonas aeruginosa*, it is **not resistant** to beta-lactamases. It is easily degraded by staphylococcal penicillinase and often requires combination with a beta-lactamase inhibitor (like clavulanic acid) to be effective against resistant strains. **Analysis of Incorrect Options:** * **A. Methicillin:** The prototype beta-lactamase resistant penicillin. It is no longer used clinically due to nephrotoxicity (interstitial nephritis) but is used in labs to define MRSA (Methicillin-Resistant *Staphylococcus aureus*). * **C. Nafcillin:** A highly lipid-soluble, penicillinase-resistant drug. It is primarily excreted via bile and is a first-line treatment for MSSA (Methicillin-Susceptible *Staphylococcus aureus*) infections like endocarditis. * **D. Oxacillin:** Along with Cloxacillin and Dicloxacillin, these are acid-stable, penicillinase-resistant drugs suitable for oral administration against staphylococcal skin and soft tissue infections. **NEET-PG High-Yield Pearls:** * **Mnemonic for Penicillinase-resistant drugs:** "**M**any **C**ans **O**f **N**ice **D**rinks" (**M**ethicillin, **C**loxacillin, **O**xacillin, **N**afcillin, **D**icloxacillin). * **MRSA Mechanism:** Resistance is due to an altered target site (**PBP-2a**), encoded by the **mecA gene**, not due to beta-lactamase production. * **Drug of Choice:** Nafcillin/Oxacillin are drugs of choice for MSSA; Vancomycin is the drug of choice for MRSA.
Question 1175: All of the following are anti-HIV agents except?
- A. Ritonavir
- B. Acyclovir (Correct Answer)
- C. Didanosine
- D. Zidovudine
Explanation: **Explanation:** The correct answer is **Acyclovir** because it is an anti-herpetic agent, not an anti-HIV agent. **1. Why Acyclovir is the correct answer:** Acyclovir is a nucleoside analog that specifically inhibits **Herpes Simplex Virus (HSV-1, HSV-2)** and **Varicella-Zoster Virus (VZV)**. It requires activation (phosphorylation) by the viral enzyme **thymidine kinase**, which is present in herpes viruses but not in HIV. Therefore, it has no clinical activity against HIV. **2. Why the other options are incorrect (Anti-HIV Agents):** * **Zidovudine (AZT):** The first drug approved for HIV. It is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)** that acts as a chain terminator during viral DNA synthesis. * **Didanosine (ddI):** Another **NRTI** used in Highly Active Antiretroviral Therapy (HAART). It is known for specific side effects like pancreatitis and peripheral neuropathy. * **Ritonavir:** A **Protease Inhibitor (PI)**. In modern medicine, it is primarily used in low doses as a "pharmacokinetic enhancer" (booster) because it inhibits the CYP3A4 enzyme, increasing the levels of other protease inhibitors. **Clinical Pearls for NEET-PG:** * **Zidovudine** is the drug of choice for preventing vertical transmission (mother-to-child) of HIV during pregnancy/labor. Its dose-limiting toxicity is **anemia/bone marrow suppression**. * **Ritonavir** is the most potent inhibitor of the Cytochrome P450 system among antiretrovirals. * **Tenofovir** (an NtRTI) is currently a backbone of first-line HAART regimens. * **Acyclovir** is the drug of choice for Herpes Simplex Encephalitis.
Question 1176: Which of the following penicillins has the best gram-negative spectrum?
- A. Methicillin
- B. Ampicillin (Correct Answer)
- C. Penicillin V
- D. Cefixime
Explanation: **Explanation:** The correct answer is **Ampicillin**. **1. Why Ampicillin is correct:** Ampicillin belongs to the **Extended-spectrum Penicillins** (Aminopenicillins). Unlike natural penicillins, which are primarily effective against Gram-positive organisms, ampicillin has an amino group that allows it to pass through the porin channels of the outer membrane of Gram-negative bacteria. This extends its spectrum to include organisms like *E. coli, Proteus mirabilis, H. influenzae,* and *Salmonella/Shigella*. **2. Analysis of Incorrect Options:** * **Methicillin:** This is a **Penicillinase-resistant penicillin**. Its clinical utility is strictly limited to penicillinase-producing *Staphylococci* (Gram-positive). It has virtually no activity against Gram-negative bacteria. * **Penicillin V:** This is an **Acid-stable natural penicillin**. Its spectrum is narrow, targeting mainly Gram-positive cocci (*Streptococci*) and some Gram-negative cocci (*Neisseria*), but it lacks activity against Gram-negative bacilli. * **Cefixime:** While Cefixime has an excellent Gram-negative spectrum, it is a **Third-generation Cephalosporin**, not a penicillin. The question specifically asks for the "penicillin" with the best spectrum. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Aminopenicillins (Ampicillin/Amoxicillin) spectrum:** **HELPS** (*H. influenzae, E. coli, Listeria, P. mirabilis, Salmonella/Shigella*). * **Drug of Choice:** Ampicillin remains the drug of choice for *Listeria monocytogenes* infections and Enterococcal endocarditis (in combination with aminoglycosides). * **Antipseudonal Penicillins:** If the options included **Piperacillin** or **Ticarcillin**, they would have an even broader Gram-negative spectrum than Ampicillin, specifically covering *Pseudomonas aeruginosa*.
Question 1177: Which of the following is NOT an antiviral drug?
- A. Vidarbine
- B. Acyclovir
- C. Zidovudine
- D. Mitomycin (Correct Answer)
Explanation: **Explanation:** The correct answer is **Mitomycin (Option D)**. **1. Why Mitomycin is the correct answer:** Mitomycin (specifically Mitomycin-C) is an **antineoplastic antibiotic** derived from *Streptomyces caespitosus*. It acts as an alkylating agent that cross-links DNA, inhibiting DNA synthesis. It is primarily used in cancer chemotherapy (e.g., stomach, pancreas, and bladder cancer) and topically in ophthalmology to prevent scarring after glaucoma surgery. It has no clinical application as an antiviral agent. **2. Why the other options are incorrect:** * **Vidarabine (Option A):** Also known as Adenine Arabinoside (ara-A), it is a purine nucleoside analogue. It was one of the first systemic antivirals used for Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV), though it is now largely replaced by safer drugs. * **Acyclovir (Option B):** A prototype guanosine analogue and a potent antiviral. It is the drug of choice for HSV and VZV. It requires viral thymidine kinase for activation (phosphorylation). * **Zidovudine (Option C):** Also known as AZT, it is a Nucleoside Reverse Transcriptase Inhibitor (NRTI). It was the first drug approved for the treatment of HIV/AIDS. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mitomycin-C Toxicity:** The dose-limiting toxicity is **delayed bone marrow suppression** (thrombocytopenia and leukopenia). * **Acyclovir Resistance:** Occurs due to the absence or mutation of the viral enzyme **thymidine kinase**. * **Zidovudine Side Effect:** Characteristically causes **macrocytic anemia** and nail hyperpigmentation. * **Antiviral vs. Antibiotic:** Many "mycin" drugs are antibacterial (e.g., Erythromycin) or antitumor (e.g., Dactinomycin), but Mitomycin is strictly an anticancer agent.
Question 1178: Which drug is strictly contraindicated in a pregnant female?
- A. Streptomycin (Correct Answer)
- B. Isoniazid
- C. Cephalosporins
- D. Penicillin
Explanation: **Explanation:** **Streptomycin** is the correct answer because it belongs to the **Aminoglycoside** class of antibiotics, which are strictly contraindicated in pregnancy. Streptomycin is classified as **FDA Pregnancy Category D**. It crosses the placental barrier and is highly **ototoxic** to the developing fetus. Exposure during pregnancy can lead to permanent **eighth cranial nerve (vestibulocochlear) damage**, resulting in congenital bilateral deafness. **Analysis of Incorrect Options:** * **Isoniazid (INH):** Generally considered safe during pregnancy. While it carries a risk of peripheral neuropathy (preventable with Pyridoxine/Vitamin B6 supplementation) and hepatotoxicity in the mother, it is not teratogenic. * **Cephalosporins:** These are **FDA Category B** drugs. They are considered safe and are often the first-line treatment for various infections in pregnant women. * **Penicillins:** Like cephalosporins, penicillins (e.g., Amoxicillin, Ampicillin) are **Category B** and are among the safest antibiotics to use during pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Safe in Pregnancy (Mnemonic: "PC"):** **P**enicillins, **C**ephalosporins, Erythromycin (except estolate form), and Azithromycin. * **Contraindicated (Mnemonic: "SAFE"):** **S**ulfonamides (Kernicterus), **A**minoglycosides (Ototoxicity), **F**luoroquinolones (Cartilage damage), **E**rythromycin estolate (Hepatotoxicity) and **T**etracyclines (Discolored teeth/bone dysplasia). * **Anti-TB Drugs in Pregnancy:** All first-line drugs (HRZE) are safe except **Streptomycin**. Ethambutol is the safest among them.
Question 1179: Which of the following statements about vancomycin is NOT true?
- A. It is a bactericidal antibiotic active primarily against gram-positive bacteria.
- B. It acts by inhibiting bacterial protein synthesis. (Correct Answer)
- C. It is an alternative to penicillin for enterococcal endocarditis.
- D. It can cause deafness as a dose-related toxicity.
Explanation: **Explanation** **1. Why Option B is the Correct Answer (The False Statement):** Vancomycin does **not** inhibit protein synthesis. Instead, it is a **cell wall synthesis inhibitor**. It acts by binding firmly to the **D-Ala-D-Ala** terminus of the nascent peptidoglycan pentapeptide. This binding sterically hinders transglycosylation and transpeptidation, preventing the cross-linking of the peptidoglycan layer. In contrast, protein synthesis inhibitors typically target the 30S or 50S ribosomal subunits (e.g., Aminoglycosides, Macrolides). **2. Analysis of Other Options:** * **Option A:** Vancomycin is indeed **bactericidal** (except against Enterococci, where it is bacteriostatic) and has a narrow spectrum limited to **Gram-positive** organisms (MRSA, *S. epidermidis*, *C. difficile*). Its large molecular size prevents it from crossing the outer membrane of Gram-negative bacteria. * **Option C:** It is a standard alternative for patients allergic to penicillin in cases of **Enterococcal endocarditis**, usually used in combination with an aminoglycoside for synergy. * **Option D:** **Ototoxicity** (deafness) and nephrotoxicity are well-known dose-related adverse effects, especially when vancomycin is administered with other ototoxic drugs like furosemide or gentamicin. **High-Yield Clinical Pearls for NEET-PG:** * **Red Man Syndrome:** A common infusion-related reaction caused by histamine release (not an allergy). Prevented by slowing the infusion rate. * **Drug of Choice:** For **MRSA** and orally for **Pseudomembranous colitis** (*C. difficile*). * **Resistance Mechanism:** Alteration of the binding site from D-Ala-D-Ala to **D-Ala-D-Lac** (seen in VRSA/VRE). * **Excretion:** Primarily renal; requires dose adjustment in renal failure.
Question 1180: A 65-year-old male with pneumonia has a sputum culture positive for a staphylococcal strain that is -lactamase-positive. Which is the best choice of penicillin therapy in this patient?
- A. Ampicillin
- B. Oxacillin (Correct Answer)
- C. Ticarcillin
- D. Penicillin G
Explanation: ### Explanation **Core Concept: Penicillinase-Resistant Penicillins** The patient is infected with a staphylococcal strain that produces **$\beta$-lactamase** (specifically penicillinase). This enzyme hydrolyzes the $\beta$-lactam ring, rendering standard penicillins inactive. To treat such infections, we must use **Penicillinase-Resistant Penicillins** (Antistaphylococcal Penicillins). These drugs have a bulky side chain that sterically hinders the $\beta$-lactamase enzyme from reaching the $\beta$-lactam ring. **Why Oxacillin is Correct:** **Oxacillin** (along with Nafcillin, Cloxacillin, and Dicloxacillin) belongs to the class of penicillinase-resistant penicillins. They are the drugs of choice for **MSSA** (Methicillin-Sensitive *Staphylococcus aureus*). Since the question specifies a $\beta$-lactamase-positive staphylococcal strain, Oxacillin is the most appropriate and narrow-spectrum choice among the options. **Why Other Options are Incorrect:** * **A. Ampicillin:** An extended-spectrum penicillin that is highly susceptible to degradation by staphylococcal $\beta$-lactamase. * **C. Ticarcillin:** An antipseudonal penicillin. While it has a broader spectrum against Gram-negative bacilli, it is destroyed by staphylococcal $\beta$-lactamase. * **D. Penicillin G:** The prototype penicillin; it is the drug of choice for sensitive Gram-positive cocci (like *S. pyogenes*) but is completely inactivated by staphylococcal penicillinase. **High-Yield Clinical Pearls for NEET-PG:** * **MRSA Mechanism:** Resistance in MRSA is **not** due to $\beta$-lactamase but due to an altered target site (**PBP-2a**), encoded by the ***mecA* gene**. * **Drug of Choice for MRSA:** Vancomycin. * **Excretion:** Nafcillin and Oxacillin are primarily excreted via **bile**, making them safe in patients with renal failure (no dose adjustment needed). * **Interstitial Nephritis:** Historically associated with Methicillin (which is why it is no longer used clinically).
Practice by Chapter
Beta-Lactam Antibiotics
Practice Questions
Aminoglycosides
Practice Questions
Macrolides and Ketolides
Practice Questions
Tetracyclines
Practice Questions
Quinolones
Practice Questions
Sulfonamides and Trimethoprim
Practice Questions
Antimycobacterial Drugs
Practice Questions
Antifungal Agents
Practice Questions
Antiviral Drugs
Practice Questions
Antiparasitic Agents
Practice Questions
Principles of Antimicrobial Selection
Practice Questions
Antimicrobial Resistance
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free