Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 521: The Jarisch-Herxheimer reaction is best managed by administering which of the following?

A. Intravenous Penicillin G
B. Antihistamines (Correct Answer)
C. Tetracyclines
D. Steroids

Explanation: **Explanation:** The **Jarisch-Herxheimer Reaction (JHR)** is a transient clinical phenomenon seen shortly after starting antibiotic treatment for spirochetal infections, most notably **Syphilis** (*Treponema pallidum*). It is caused by the sudden release of endotoxins and lipoproteins (like cytokines TNF-α, IL-6, and IL-8) from the dying spirochetes. **Why Antihistamines are correct:** The management of JHR is primarily **symptomatic and supportive**. While the reaction can be alarming (fever, chills, headache, and exacerbation of skin lesions), it is usually self-limiting (12–24 hours). **Antihistamines** and antipyretics (like Aspirin or NSAIDs) are administered to control the inflammatory symptoms and provide symptomatic relief. **Why other options are incorrect:** * **A. Intravenous Penicillin G:** This is the *trigger* for the reaction, not the treatment. Continuing or increasing the dose of the antibiotic does not stop the toxin-mediated inflammatory cascade already in progress. * **C. Tetracyclines:** Like Penicillin, these are antibiotics that can actually induce JHR when treating conditions like Lyme disease or Relapsing fever. * **D. Steroids:** While steroids (like Prednisolone) can be used to *prevent* JHR in specific high-risk cases (e.g., Neurosyphilis or Cardiovascular syphilis to prevent a "therapeutic shock"), they are not the standard first-line management for a reaction that has already commenced. **Clinical Pearls for NEET-PG:** * **Most Common Association:** Secondary Syphilis treated with Penicillin. * **Timing:** Usually occurs within 2–12 hours of the first antibiotic dose. * **Key Mediator:** Tumor Necrosis Factor-alpha (TNF-α). * **Important Distinction:** JHR is **not** a Penicillin allergy. Treatment should not be discontinued; the patient should be reassured and treated symptomatically.

Question 522: Which of the following statements is true about the antiviral drug Remdesivir?

A. It was developed during the MERS outbreak in 2012.
B. It is an adenosine analog that interferes with viral RNA-dependent RNA polymerase, leading to decreased viral RNA production. (Correct Answer)
C. It is the drug of choice for treating pneumonia caused by SARS-CoV-2.
D. It is an FDA-approved drug for COVID-19.

Explanation: **Explanation:** **Mechanism of Action (Why B is Correct):** Remdesivir is a **nucleoside analog** (specifically an **adenosine analog**). It acts as a prodrug that, once inside the cell, is metabolized into its active triphosphate form. This active metabolite competes with ATP for incorporation into the nascent viral RNA strand by the **viral RNA-dependent RNA polymerase (RdRp)**. Once incorporated, it causes **delayed chain termination**, effectively halting viral replication and decreasing viral RNA production. **Analysis of Incorrect Options:** * **Option A:** Remdesivir was originally developed by Gilead Sciences to treat **Ebola virus** and Marburg virus infections, not during the 2012 MERS outbreak (though it later showed in-vitro activity against MERS). * **Option C:** While used in treatment protocols, it is **not the "drug of choice"** for pneumonia. Current guidelines (WHO/NIH) prioritize corticosteroids (Dexamethasone) and immunomodulators (Tocilizumab) for severe pneumonia, as Remdesivir primarily benefits patients requiring supplemental oxygen but not yet on high-flow or mechanical ventilation. * **Option D:** While Remdesivir (Veklury) did receive FDA approval, in the context of NEET-PG multiple-choice questions, the **pharmacological mechanism** (Option B) is considered the most "scientifically true" and definitive statement compared to regulatory status, which can vary by region and time. **High-Yield Clinical Pearls for NEET-PG:** * **Route of Administration:** Given only **Intravenously (IV)**; not absorbed orally. * **Adverse Effects:** Most common are transaminase elevation (hepatotoxicity) and increased prothrombin time. * **Contraindication:** Generally avoided if eGFR < 30 mL/min due to the accumulation of the vehicle (Sulfobutylether-β-cyclodextrin). * **Spectrum:** Broad-spectrum antiviral activity against Coronaviridae, Filoviridae (Ebola), and Paramyxoviridae.

Question 523: Which of the following drugs acts on the 50s ribosomal subunit?

A. Streptomycin
B. Tetracycline
C. Quinupristin (Correct Answer)
D. Demeclocycline

Explanation: **Explanation:** Protein synthesis inhibitors are a high-yield topic in NEET-PG, classified based on whether they bind to the 30S or 50S ribosomal subunit. **Why Quinupristin is Correct:** **Quinupristin** (along with Dalfopristin) belongs to the **Streptogramin** class. These drugs act on the **50S ribosomal subunit**. Specifically, Quinupristin inhibits peptide chain elongation and promotes premature detachment of the peptide chain. When used in a 30:70 combination (Synercid), they act synergistically to become bactericidal against Gram-positive cocci, including MRSA and VRE. **Why Other Options are Incorrect:** * **Streptomycin (Option A):** This is an Aminoglycoside. Aminoglycosides bind to the **30S subunit**, causing mRNA misreading and inhibition of the initiation complex. * **Tetracycline (Option B) & Demeclocycline (Option D):** Both belong to the Tetracycline class. These drugs bind reversibly to the **30S subunit**, preventing the attachment of aminoacyl-tRNA to the 'A' site. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for 50S Inhibitors:** **"CCEL"** (**C**hloramphenicol, **C**lindamycin, **E**rythromycin/Macrolides, **L**inezolid) + Streptogramins. * **Mnemonic for 30S Inhibitors:** **"AT"** (**A**minoglycosides, **T**etracyclines). * **Demeclocycline** is unique because it is also used to treat **SIADH** as it inhibits ADH action in the renal tubules. * **Linezolid** (50S) is a notable cause of **Serotonin Syndrome** when combined with SSRIs due to its weak MAO-inhibitory activity.

Question 524: What is the drug of choice for herpes simplex virus infection?

A. Acyclovir (Correct Answer)
B. Zidovudine
C. Indinavir
D. Ribavarin

Explanation: **Explanation:** **Acyclovir** is the drug of choice (DOC) for infections caused by **Herpes Simplex Virus (HSV-1, HSV-2)** and Varicella-Zoster Virus (VZV). It is a guanosine analogue that acts as a "prodrug." Its selectivity stems from the fact that it must first be phosphorylated into acyclovir monophosphate by the viral enzyme **Thymidine Kinase**. Host cell enzymes then convert it to acyclovir triphosphate, which inhibits viral DNA polymerase and causes DNA chain termination. Because it requires the viral enzyme for activation, it has low toxicity to healthy human cells. **Analysis of Incorrect Options:** * **B. Zidovudine (AZT):** A Nucleoside Reverse Transcriptase Inhibitor (NRTI) used primarily in the treatment of **HIV/AIDS**. It targets the reverse transcriptase enzyme, which HSV does not possess. * **C. Indinavir:** A **Protease Inhibitor** used in Highly Active Antiretroviral Therapy (HAART) for **HIV**. It prevents the cleavage of viral polyproteins into functional units. * **D. Ribavirin:** A broad-spectrum antiviral used primarily for **Hepatitis C** (in combination with interferon) and **Respiratory Syncytial Virus (RSV)** in children. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Resistance to acyclovir occurs due to the mutation or absence of the viral **Thymidine Kinase** enzyme. * **DOC for HSV Encephalitis:** Intravenous Acyclovir is the gold standard. * **Side Effects:** The most significant side effect of IV acyclovir is **crystalline nephropathy**; ensure adequate hydration to prevent renal damage. * **Valacyclovir:** A prodrug of acyclovir with better oral bioavailability, often preferred for outpatient management of shingles or genital herpes.

Question 525: Which of the following drugs is used in the treatment of acute bacterial meningitis?

A. Erythromycin
B. Sulfamethoxazole
C. Ceftriaxone (Correct Answer)
D. Streptomycin

Explanation: ### Explanation **Correct Option: C. Ceftriaxone** The primary requirement for treating acute bacterial meningitis is the ability of the drug to cross the **blood-brain barrier (BBB)** and achieve therapeutic concentrations in the cerebrospinal fluid (CSF). **Ceftriaxone** (a 3rd-generation cephalosporin) is the drug of choice because it has excellent CSF penetration, especially when the meninges are inflamed. It also possesses a broad spectrum of activity against common causative organisms like *Streptococcus pneumoniae* and *Neisseria meningitidis*. **Analysis of Incorrect Options:** * **A. Erythromycin:** This macrolide has very poor CNS penetration and is primarily used for respiratory and atypical infections. It is not indicated for meningitis. * **B. Sulfamethoxazole:** While sulfonamides can cross the BBB, they are bacteriostatic and have high resistance rates. They are rarely used alone for acute pyogenic meningitis. * **D. Streptomycin:** This aminoglycoside is highly polar and does not cross the BBB effectively. Its use is limited to specific infections like Tuberculosis or Plague. **NEET-PG High-Yield Pearls:** 1. **Empiric Therapy:** The standard initial treatment for adult bacterial meningitis is **Ceftriaxone + Vancomycin** (to cover penicillin-resistant *S. pneumoniae*). 2. **Listeria Coverage:** In neonates or elderly patients, **Ampicillin** must be added to cover *Listeria monocytogenes*. 3. **Steroid Adjunct:** Dexamethasone is often administered just before or with the first dose of antibiotics to reduce neurological complications (like hearing loss) caused by inflammatory cytokines. 4. **Excretion:** Unlike most cephalosporins, Ceftriaxone is primarily excreted via **bile**, making it safer in patients with renal failure.

Question 526: Mebendazole cannot be used for which of the following conditions?

A. Ascariasis
B. Enterobius vermicularis infection
C. Onchocercosis (Correct Answer)
D. Hydatid cyst disease

Explanation: Explanation: The correct answer is Onchocercosis (River Blindness). 1. Why Onchocercosis is the correct answer: Mebendazole is a broad-spectrum anthelmintic belonging to the Benzimidazole group. Its primary mechanism of action is the inhibition of microtubule synthesis by binding to β-tubulin, which leads to glucose depletion and death of the parasite [2]. While it is highly effective against intestinal nematodes, it has poor systemic bioavailability. Onchocercosis is caused by Onchocerca volvulus, a tissue-dwelling nematode [4, 5]. The drug of choice for Onchocercosis is Ivermectin (which targets microfilariae) or Doxycycline (which targets the Wolbachia symbiont). Mebendazole is not used because it lacks sufficient efficacy against these tissue filariae. 2. Analysis of incorrect options: * Ascariasis (Roundworm): Mebendazole is a first-line agent for Ascaris lumbricoides [3]. It causes slow immobilization and expulsion of the worms. * Enterobius vermicularis (Pinworm): Mebendazole is highly effective for pinworm infections [3]. A single 100 mg dose, repeated after two weeks, is the standard regimen. * Hydatid cyst disease: Caused by Echinococcus granulosus. While Albendazole is the preferred benzimidazole due to better tissue penetration, Mebendazole is an alternative treatment, especially when Albendazole is not tolerated [2]. 3. Clinical Pearls for NEET-PG: * Albendazole vs. Mebendazole: Albendazole is generally preferred for systemic/tissue infections (like Neurocysticercosis and Hydatid disease) because its absorption is enhanced by a fatty meal [2]. * Teratogenicity: Benzimidazoles are generally avoided during the first trimester of pregnancy [1]. * Drug of Choice (DOC) Summary: * Strongyloidiasis & Onchocerciasis: Ivermectin [3] * Neurocysticercosis & Hydatid Cyst: Albendazole [2] * Filariasis (W. bancrofti): Diethylcarbamazine (DEC)

Question 527: Which drug is used for topical application per vaginum?

A. Clotrimazole
B. Miconazole
C. Nystatin
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above**. This question tests the knowledge of topical antifungal therapy for **Vulvovaginal Candidiasis (VVC)**, a common clinical condition caused primarily by *Candida albicans*. **1. Why the correct answer is right:** All three drugs listed belong to classes of antifungals that are poorly absorbed systemically when applied to mucous membranes, making them ideal for local (topical) application. * **Clotrimazole and Miconazole** are **Imidazoles**. They inhibit the enzyme *14-alpha-demethylase*, preventing the synthesis of ergosterol (a key component of the fungal cell membrane). They are available as vaginal creams and pessaries (suppositories). * **Nystatin** is a **Polyene** antibiotic (similar to Amphotericin B). It binds to ergosterol in the fungal cell membrane, creating pores that lead to cell death. It is specifically used for candidal infections and is not absorbed from the skin or vagina. **2. Analysis of Options:** * **Clotrimazole:** Often the first-line topical choice. High-yield fact: It is also used for oropharyngeal candidiasis (troches). * **Miconazole:** Frequently used in over-the-counter vaginal creams and suppositories. * **Nystatin:** While less commonly used now due to the high efficacy of azoles, it remains a classic topical treatment for vaginal thrush. **3. Clinical Pearls for NEET-PG:** * **Drug of Choice:** For VVC, a single dose of **Oral Fluconazole (150 mg)** is often preferred for patient convenience, but topical azoles are equally effective. * **Pregnancy:** Topical imidazoles (like Clotrimazole) are the **preferred treatment** for VVC during pregnancy; oral fluconazole is generally avoided in the first trimester. * **Mechanism Recap:** Azoles = Inhibition of ergosterol *synthesis*; Polyenes = *Binding* to ergosterol.

Question 528: Which is the safest anti-tuberculosis drug to use during pregnancy?

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

Explanation: **Explanation:** The management of Tuberculosis (TB) during pregnancy follows the standard WHO-recommended regimen, but safety profiles vary among drugs. **Isoniazid (INH)** is considered the safest and most preferred anti-TB drug in pregnancy. It is classified as FDA Category C but has a long-standing record of safety without documented teratogenicity. When administering INH to a pregnant woman, it is mandatory to co-administer **Pyridoxine (Vitamin B6)** to prevent peripheral neuropathy in both the mother and the fetus. **Analysis of Options:** * **Isoniazid (INH):** Correct. It is the cornerstone of TB treatment in pregnancy. While it carries a risk of hepatotoxicity, it does not cause fetal malformations. * **Rifampicin:** Generally considered safe and used in the standard 4-drug regimen (RHE), but it carries a theoretical risk of neonatal hemorrhage due to Vitamin K antagonism. * **Ethambutol:** Considered safe and non-teratogenic; however, INH remains the primary drug of choice for safety profiles in most clinical guidelines. * **Streptomycin:** **Absolutely contraindicated.** It is an aminoglycoside that causes permanent **ototoxicity** (8th cranial nerve damage) and nephrotoxicity in the fetus, leading to congenital deafness. **Clinical Pearls for NEET-PG:** * **Standard Regimen:** The preferred treatment for TB in pregnancy is 2 months of HRZE followed by 4 months of HR. * **Pyrazinamide:** While the WHO recommends its use, some older US guidelines (ATS) traditionally avoided it due to limited safety data; however, it is now widely accepted as safe. * **Contraindicated TB Drugs:** Streptomycin (Ototoxicity) and Ethionamide (Teratogenic). * **Breastfeeding:** All first-line anti-TB drugs (HRZE) are compatible with breastfeeding as they are excreted in negligible amounts in breast milk.

Question 529: Which of the following is NOT an indication for Metronidazole?

A. Ulcerative gingivitis
B. Amoebiasis
C. Trichomonas infection
D. Streptococcal sore throat (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Streptococcal sore throat.** **Mechanism and Spectrum of Action:** Metronidazole is a nitroimidazole derivative that acts by forming reactive cytotoxic intermediates (free radicals) that damage bacterial DNA. This process occurs only under **anaerobic conditions**. Therefore, Metronidazole is highly effective against **obligate anaerobes** and certain **protozoa**, but it has **no activity against aerobic bacteria**. 1. **Streptococcal sore throat (Option D):** This is caused by *Streptococcus pyogenes* (Group A Streptococcus), which is a Gram-positive aerobe/facultative anaerobe. Since Metronidazole lacks activity against aerobic organisms, it is ineffective. The drug of choice for Streptococcal pharyngitis remains Penicillin V or Amoxicillin. **Why other options are incorrect:** * **Ulcerative gingivitis (Option A):** Acute Necrotizing Ulcerative Gingivitis (ANUG) involves anaerobic fusiform bacteria and spirochetes. Metronidazole is a first-line agent here. * **Amoebiasis (Option B):** Metronidazole is the drug of choice for intestinal and extra-intestinal (liver abscess) amoebiasis caused by *Entamoeba histolytica*. * **Trichomonas infection (Option C):** It is the gold standard treatment for *Trichomonas vaginalis* (Trichomoniasis) in both symptomatic patients and their sexual partners. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for:** *Clostridioides difficile* (mild-to-moderate), Bacterial vaginosis, and Giardiasis. * **Disulfiram-like reaction:** Patients must avoid alcohol while on Metronidazole due to the inhibition of aldehyde dehydrogenase. * **Side Effects:** Metallic taste (most common), peripheral neuropathy (prolonged use), and seizures. * **Triple Therapy:** It is a component of the regimen for *H. pylori* eradication.

Question 530: All of the following are side effects of Linezolid, EXCEPT?

A. Optic neuropathy
B. Pancytopenia
C. Cardiac arrhythmia (Correct Answer)
D. Lactic acidosis

Explanation: **Explanation:** Linezolid is an oxazolidinone antibiotic used primarily for resistant Gram-positive infections like MRSA and VRE. It acts by inhibiting the formation of the 70S initiation complex. **Why Cardiac Arrhythmia is the correct answer:** Linezolid is **not** associated with cardiac arrhythmias or QT interval prolongation. Its primary toxicities are related to mitochondrial protein synthesis inhibition (due to the similarity between bacterial and mitochondrial ribosomes) and its role as a weak, reversible non-selective Monoamine Oxidase Inhibitor (MAOI). **Analysis of Incorrect Options:** * **Optic Neuropathy:** Prolonged use (usually >28 days) can lead to mitochondrial dysfunction in the optic nerve, resulting in optic neuropathy and potential vision loss. Peripheral neuropathy is also a known risk. * **Pancytopenia:** Linezolid causes time-dependent bone marrow suppression. Thrombocytopenia is the most common manifestation, but it can progress to anemia, leukopenia, and full pancytopenia. Monitoring CBC weekly is recommended. * **Lactic Acidosis:** This is a serious metabolic side effect resulting from Linezolid’s interference with mitochondrial oxidative phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Serotonin Syndrome:** Because Linezolid is a weak MAOI, it can cause Serotonin Syndrome if co-administered with SSRIs or other serotonergic drugs. 2. **Mechanism of Action:** Unique binding site on the **23S rRNA of the 50S subunit**, preventing the formation of the 70S initiation complex. 3. **Bioavailability:** It has **100% oral bioavailability**, allowing for an easy IV-to-oral switch. 4. **Drug of Choice:** Often used for Vancomycin-resistant *Enterococcus faecium* (VRE) and Nosocomial pneumonia caused by MRSA.

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

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