Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 741: Which of the following is an oral cephalosporin?

A. Cefatoxime
B. Ceftriaxone
C. Cefaclor (Correct Answer)
D. Ceftazidime

Explanation: ### Explanation **Correct Option: C. Cefaclor** Cephalosporins are classified into generations based on their antimicrobial spectrum and route of administration. **Cefaclor** is a **Second-Generation Cephalosporin** that is primarily administered **orally**. It is frequently used in clinical practice for mild-to-moderate infections like otitis media and respiratory tract infections due to its activity against *H. influenzae* and *Moraxella catarrhalis*. **Analysis of Incorrect Options:** * **A. Cefotaxime:** A **Third-Generation Cephalosporin** administered only via **parenteral** (IV/IM) routes. It is a mainstay for treating meningitis due to its excellent CSF penetration. * **B. Ceftriaxone:** A **Third-Generation Cephalosporin** administered **parenterally**. It is notable for its long half-life (allowing once-daily dosing) and biliary excretion. * **D. Ceftazidime:** A **Third-Generation Cephalosporin** administered **parenterally**. It is highly specific for its potent activity against ***Pseudomonas aeruginosa***. **High-Yield NEET-PG Pearls:** 1. **Oral Cephalosporins Mnemonic:** Remember "The **Oral** trio": **Cephalexin** (1st Gen), **Cefaclor/Cefuroxime axetil** (2nd Gen), and **Cefixime/Cefdinir** (3rd Gen). 2. **Cefuroxime Paradox:** Cefuroxime is unique as it is available in both parenteral (Cefuroxime sodium) and oral (Cefuroxime axetil) forms. 3. **Cefixime** is the most commonly used **oral** 3rd generation cephalosporin, often used for uncomplicated urinary tract infections and switch therapy from IV ceftriaxone. 4. **Excretion:** Most cephalosporins are renally excreted; **Ceftriaxone** and **Cefoperazone** are exceptions as they are primarily excreted in the **bile**.

Question 742: Lamivudine acts against which of the following viruses?

A. HIV
B. HBV
C. Both HIV and HBV (Correct Answer)
D. None of the above

Explanation: **Explanation:** Lamivudine (3TC) is a nucleoside analog that acts as a potent inhibitor of the enzyme **Reverse Transcriptase**. Its mechanism involves being phosphorylated intracellularly into its active triphosphate form, which then competes with natural nucleotides for incorporation into viral DNA, leading to **obligate chain termination**. **Why Option C is Correct:** * **HIV:** Lamivudine is a cornerstone of Highly Active Antiretroviral Therapy (HAART). It belongs to the **NRTI (Nucleoside Reverse Transcriptase Inhibitor)** class and is effective against both HIV-1 and HIV-2. * **HBV:** The Hepatitis B Virus, though a DNA virus, utilizes a reverse transcription step in its replication cycle (via HBV DNA polymerase). Lamivudine effectively inhibits this enzyme, making it one of the first oral drugs approved for chronic Hepatitis B. **Why other options are incorrect:** * **Options A & B:** These are partially correct but incomplete. Since Lamivudine is clinically indicated and FDA-approved for both infections, "Both" is the most accurate choice for a competitive exam. **High-Yield Clinical Pearls for NEET-PG:** * **Dosing Difference:** The dose of Lamivudine required for HBV (100 mg/day) is lower than that required for HIV (300 mg/day). * **M184V Mutation:** This is the specific high-level resistance mutation associated with Lamivudine in HIV treatment. * **Safety:** It is one of the least toxic NRTIs; however, it can cause a "flare-up" of Hepatitis B if discontinued abruptly in co-infected patients. * **Pregnancy:** It is considered safe and is frequently used to prevent vertical transmission of HIV.

Question 743: Which third-generation cephalosporin can be administered orally?

A. Cefixime (Correct Answer)
B. Cefpirome
C. Cefaclor
D. Cefadroxil

Explanation: ### Explanation **Correct Answer: A. Cefixime** **1. Why Cefixime is correct:** Cephalosporins are classified into generations based on their chronological development and spectrum of activity. **Cefixime** is a prototype **third-generation cephalosporin** that is characterized by its stability against many beta-lactamases and its availability as an **oral formulation**. It is highly effective against Gram-negative organisms (like *H. influenzae* and *M. catarrhalis*) but has limited activity against *S. aureus*. **2. Analysis of Incorrect Options:** * **B. Cefpirome:** This is a **fourth-generation** cephalosporin. It is highly resistant to beta-lactamases and is administered only via the parenteral (IV/IM) route for serious hospital-acquired infections. * **C. Cefaclor:** This is a **second-generation** cephalosporin. While it is administered orally, it belongs to an earlier generation with a different spectrum of activity (better Gram-positive coverage than 3rd gen). * **D. Cefadroxil:** This is a **first-generation** cephalosporin. It is an oral agent primarily used for skin and soft tissue infections caused by Gram-positive cocci. **3. NEET-PG High-Yield Pearls:** * **Oral 3rd Gen Cephalosporins:** Besides Cefixime, other oral options include **Cefpodoxime proxetil**, **Ceftibuten**, and **Cefdinir**. * **Cefixime Clinical Use:** It is a drug of choice for oral treatment of uncomplicated urinary tract infections and was traditionally used for gonorrhea (though resistance is increasing). * **Excretion:** Unlike most cephalosporins which are renally excreted, **Ceftriaxone** (3rd gen) is primarily excreted through bile and does not require dose adjustment in renal failure. * **Anti-Pseudomonal 3rd Gen:** **Ceftazidime** and **Cefoperazone** are the third-generation agents with specific activity against *Pseudomonas aeruginosa*.

Question 744: What are the common complications associated with zidovudine therapy?

A. Nausea and vomiting (Correct Answer)
B. Anemia
C. Steatosis
D. Nephrotoxicity

Explanation: **Explanation:** **Zidovudine (AZT)** is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in the management of HIV. Understanding its side effect profile is crucial for NEET-PG, as it distinguishes between "common" and "dose-limiting" toxicities. 1. **Why Option A is Correct:** **Nausea and vomiting** are the most **common** (frequent) side effects of zidovudine, occurring in approximately 25% of patients, especially during the first few weeks of therapy. These gastrointestinal symptoms are usually self-limiting and diminish as the body adjusts to the medication. 2. **Why Other Options are Incorrect:** * **B. Anemia:** While zidovudine is notorious for causing bone marrow suppression (anemia and neutropenia), this is considered the most important **dose-limiting** toxicity rather than the most common initial complaint. * **C. Steatosis:** Lactic acidosis with hepatic steatosis is a class-wide adverse effect of NRTIs due to mitochondrial toxicity. While serious, it is less common than acute GI distress. * **D. Nephrotoxicity:** Zidovudine is primarily metabolized by the liver (glucuronidation). Nephrotoxicity is more characteristic of other HIV drugs like **Tenofovir** (Fanconi-like syndrome) or Indinavir (crystalluria). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Zidovudine:** "Z" for **Z**eroing out the marrow (Anemia/Neutropenia). * **Drug of Choice:** Zidovudine is the preferred agent for preventing **mother-to-child transmission (MTCT)** of HIV during pregnancy and labor. * **Nail Hyperpigmentation:** A unique, high-yield physical finding associated with chronic zidovudine use. * **MCV Change:** An increase in Mean Corpuscular Volume (macrocytosis) is a common surrogate marker for patient compliance with zidovudine.

Question 745: What is the drug of choice for Chlamydia in pregnancy?

A. Doxycycline
B. Tetracycline
C. Erythromycin (Correct Answer)
D. Penicillin

Explanation: **Explanation:** The drug of choice for treating *Chlamydia trachomatis* infections during pregnancy is **Erythromycin** (or alternatively, Azithromycin). **1. Why Erythromycin is Correct:** Chlamydia is an obligate intracellular bacterium that lacks a peptidoglycan cell wall, making it inherently resistant to beta-lactams. Macrolides like Erythromycin inhibit protein synthesis by binding to the 50S ribosomal subunit. In pregnancy, Erythromycin is considered safe (FDA Category B) and effective for preventing neonatal transmission (ophthalmia neonatorum) and maternal complications. While Azithromycin is now often preferred due to a single-dose regimen and better GI tolerance, Erythromycin remains a classic, established answer in standard textbooks and exams. **2. Why Other Options are Incorrect:** * **Doxycycline & Tetracycline:** These are the drugs of choice for Chlamydia in non-pregnant adults. However, they are **contraindicated in pregnancy** (Category D) because they cross the placenta and bind to calcium in fetal bones and teeth, leading to permanent dental discoloration and enamel hypoplasia. * **Penicillin:** As mentioned, Chlamydia lacks a typical cell wall structure. Penicillins target cell wall synthesis and are therefore clinically ineffective against this organism. **Clinical Pearls for NEET-PG:** * **Azithromycin** is currently the preferred macrolide in many guidelines due to better compliance (1g single dose). * **Neonatal Chlamydia:** Presents as inclusion conjunctivitis (5–14 days post-delivery) or interstitial pneumonia. * **Lymphogranuloma Venereum (LGV):** Caused by Chlamydia serotypes L1-L3; Doxycycline is the first-line treatment (except in pregnancy). * **Safe in Pregnancy Mnemonic:** "Many Antibiotics Can Be Safely Given" — **M**acrolides (Erythromycin/Azithromycin), **A**moxicillin, **C**ephalosporins, **B**eta-lactams, **S**pectinomycin, **G**entamicin (topical).

Question 746: Which of the following drugs is FDA-approved for the treatment of HIV?

A. Ivacaftor
B. Apalutamide
C. Ibalizumab (Correct Answer)
D. Eculizumab

Explanation: ### Explanation **Correct Option: C. Ibalizumab** Ibalizumab is a first-in-class **post-attachment inhibitor**. It is a humanized monoclonal antibody that binds to domain 2 of the CD4 T-cell receptor. Unlike entry inhibitors like Maraviroc (which targets CCR5), Ibalizumab does not prevent HIV from binding to the CD4 cell; instead, it prevents the conformational changes required for the viral envelope to fuse with the host cell membrane. It was FDA-approved in 2018 for **multidrug-resistant (MDR) HIV-1** in treatment-experienced adults. **Incorrect Options:** * **A. Ivacaftor:** A CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) potentiator used in the treatment of **Cystic Fibrosis** (specifically for the G551D mutation). * **B. Apalutamide:** A second-generation **androgen receptor inhibitor** used in the management of non-metastatic castration-resistant prostate cancer. * **C. Eculizumab:** A monoclonal antibody that inhibits the **C5 complement protein**. It is used for Paroxysmal Nocturnal Hemoglobinuria (PNH) and Atypical Hemolytic Uremic Syndrome (aHUS). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Ibalizumab blocks "post-binding" steps required for viral entry. * **Route:** Administered via **intravenous (IV) infusion** every 2 weeks (unique among most ARVs). * **Indication:** Reserved for "salvage therapy" in patients failing their current antiretroviral regimen. * **Other Entry Inhibitors:** * *Maraviroc:* CCR5 Antagonist (requires Tropism testing). * *Enfuvirtide:* Fusion inhibitor (binds to gp41). * *Fostemsavir:* Attachment inhibitor (binds to gp120).

Question 747: What is the recommended dose of chloroquine for children aged 4-8 years?

A. 300 mg (Correct Answer)
B. 150 mg
C. 450 mg
D. 600 mg

Explanation: **Explanation:** The treatment of uncomplicated malaria with Chloroquine follows a standardized weight-based or age-based regimen. For pediatric patients, the total dose is **25 mg/kg** of chloroquine base spread over three days (10 mg/kg on Day 1 & 2, and 5 mg/kg on Day 3). **Why 300 mg is correct:** In the context of age-based dosing (often used in public health programs like NVBDCP), children are categorized into specific brackets. For the **4–8 years** age group, the recommended dose is **300 mg** (base). This typically corresponds to 2 tablets of 150 mg chloroquine base. **Analysis of Incorrect Options:** * **150 mg (Option B):** This is the dose for children aged **1–4 years**. It is also the standard strength of one "base" tablet. * **450 mg (Option C):** This dose is reserved for children aged **9–14 years** (3 tablets). * **600 mg (Option D):** This is the standard adult loading dose (4 tablets) for a 60 kg adult on Day 1 of treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits heme polymerase (biocrystallization), leading to the accumulation of toxic heme in the parasite food vacuole. * **Drug of Choice:** Still the DOC for sensitive *P. falciparum* and all *P. vivax* (except in resistant pockets). It is also the DOC for malaria in **pregnancy**. * **Adverse Effects:** Pruritus (common in dark-skinned individuals), bull’s eye retinopathy (chronic use), and QTc prolongation. * **Resistance:** Mediated by the **pfcrt** gene (Chloroquine Resistance Transporter), which pumps the drug out of the parasite's food vacuole.

Question 748: What is the treatment of choice for late cardiovascular syphilis?

A. Benzathine penicillin 7.2 million units in three divided doses (Correct Answer)
B. Benzathine penicillin 2.4 million units single dose
C. Benzyl penicillin 12-24 million units for 21 days
D. Tetracycline 2 g daily

Explanation: The treatment of choice for syphilis is **Penicillin G**, as *Treponema pallidum* remains exquisitely sensitive to it [1]. The specific regimen depends on the stage and duration of the infection. **Why Option A is Correct:** Late cardiovascular syphilis (along with late latent syphilis and gummatous syphilis) is characterized by a slow multiplication rate of the spirochetes. To ensure eradication, treponemicidal levels of penicillin must be maintained in the serum for at least 2–3 weeks. **Benzathine Penicillin G (2.4 million units IM)** is administered once weekly for **three consecutive weeks**, totaling **7.2 million units**. This provides the necessary sustained-release profile required for late-stage disease. **Analysis of Incorrect Options:** * **Option B:** 2.4 million units as a single dose is the standard treatment for **Early Syphilis** (Primary, Secondary, or Early Latent <1 year). * **Option C:** High-dose IV Benzyl Penicillin (18–24 million units daily) is the treatment of choice for **Neurosyphilis**, as Benzathine penicillin does not achieve adequate levels in the CSF. * **Option D:** Tetracyclines (or Doxycycline) are used only as second-line alternatives in penicillin-allergic non-pregnant patients, but they are not the "treatment of choice." **High-Yield Clinical Pearls for NEET-PG:** * **Jarisch-Herxheimer Reaction:** A common systemic reaction (fever, headache, myalgia) occurring within hours of starting treatment, caused by the release of endotoxins from dying spirochetes. It is most common in secondary syphilis. * **Pregnancy:** Penicillin is the only recommended treatment. If the patient is allergic, they must undergo **desensitization** rather than using alternatives like Doxycycline (which is contraindicated). * **Procaine Penicillin:** No longer the first-line choice for syphilis due to the convenience of Benzathine Penicillin.

Question 749: Which of the following beta-lactam antibiotics can be safely used in a patient with a history of allergy to penicillins?

A. Aztreonam (Correct Answer)
B. Cefepime
C. Loracarbef
D. Ceftriaxone

Explanation: **Explanation:** The correct answer is **Aztreonam**. **1. Why Aztreonam is correct:** Aztreonam is a **Monobactam**. Unlike other beta-lactams, it has a monocyclic ring structure rather than a fused bicyclic ring. This structural difference means it does not exhibit cross-reactivity with penicillins or cephalosporins. It is the only beta-lactam antibiotic that can be safely administered to patients with a documented Type-1 hypersensitivity (anaphylaxis) to penicillins. *Note:* The only exception is a specific cross-reactivity between Aztreonam and **Ceftazidime**, as they share an identical side chain. **2. Why the other options are incorrect:** * **Cefepime (Option B) & Ceftriaxone (Option D):** These are 4th and 3rd generation cephalosporins, respectively. Cephalosporins share a similar bicyclic nucleus with penicillins. In patients with a history of penicillin allergy, there is a 1–10% risk of cross-reactivity. They are generally avoided if the patient has a history of immediate hypersensitivity (IgE-mediated). * **Loracarbef (Option C):** This is a Carbacephem (structurally similar to cephalosporins). It also carries a risk of cross-reactivity and is contraindicated in patients with severe penicillin allergies. **3. NEET-PG High-Yield Pearls:** * **Spectrum:** Aztreonam is active **only against aerobic Gram-negative bacteria** (including *Pseudomonas*). It has no activity against Gram-positives or anaerobes ("Gram-negative specialist"). * **Mechanism:** It binds specifically to Penicillin-Binding Protein 3 (PBP-3). * **Safety:** It is non-nephrotoxic, making it a useful alternative to aminoglycosides in penicillin-allergic patients with renal impairment.

Question 750: Which of the following is a protease inhibitor?

A. Abacavir
B. Nevirapine
C. Saquinavir (Correct Answer)
D. Enfuvirtide

Explanation: **Explanation:** **Correct Option: C. Saquinavir** Saquinavir is the correct answer as it belongs to the **Protease Inhibitor (PI)** class of antiretroviral drugs. Protease inhibitors work by inhibiting the viral enzyme **HIV-1 protease**, which is responsible for cleaving the large gag-pol polyprotein precursor into functional structural proteins and enzymes. By blocking this step, PIs prevent the maturation of viral particles, resulting in the production of immature, non-infectious virions. **Analysis of Incorrect Options:** * **A. Abacavir:** This is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)**. It acts as a competitive inhibitor of HIV reverse transcriptase and requires intracellular phosphorylation to its active triphosphate form. (Note: Associated with HLA-B*5701 hypersensitivity). * **B. Nevirapine:** This is a **Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI)**. Unlike NRTIs, it binds directly to the reverse transcriptase enzyme at a non-catalytic site to induce a conformational change. * **D. Enfuvirtide:** This is a **Fusion Inhibitor**. It binds to the gp41 subunit of the viral envelope glycoprotein, preventing the fusion of the HIV-1 envelope with the host cell membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Suffix Rule:** Most Protease Inhibitors end in the suffix **"-navir"** (e.g., Ritonavir, Indinavir, Lopinavir). * **Metabolic Side Effects:** PIs are classically associated with **lipodystrophy** (buffalo hump), hyperlipidemia, insulin resistance, and spontaneous bleeding in hemophiliacs. * **Ritonavir Boosting:** Ritonavir is a potent **CYP3A4 inhibitor**. It is often used in low doses to "boost" the plasma concentrations of other PIs, allowing for less frequent dosing. * **Indinavir** is notorious for causing **nephrolithiasis** (crystalluria).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

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Macrolides and Ketolides

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Tetracyclines

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Quinolones

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Sulfonamides and Trimethoprim

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Antimycobacterial Drugs

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Antifungal Agents

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Antiviral Drugs

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Antiparasitic Agents

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Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

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