Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 691: Which of the following antitubercular drugs acts by inhibiting bacterial DNA-dependent RNA polymerase?

A. Chloramphenicol
B. Erythromycin
C. Tetracyclines
D. Rifampicin (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Rifampicin** Rifampicin is a bactericidal antitubercular drug that belongs to the Rifamycin group. Its primary mechanism of action is the **inhibition of DNA-dependent RNA polymerase (DDRP)**. It binds to the $\beta$-subunit of this enzyme, preventing the initiation of mRNA synthesis (transcription). Since it targets the transcription process, it is highly effective against both actively dividing and "persister" (dormant) bacilli. **Analysis of Incorrect Options:** * **A. Chloramphenicol:** This is a broad-spectrum antibiotic that inhibits protein synthesis by binding to the **50S ribosomal subunit** and inhibiting the enzyme peptidyl transferase. * **B. Erythromycin:** A macrolide antibiotic that also acts on the **50S ribosomal subunit**, specifically inhibiting the translocation step of protein synthesis. * **C. Tetracyclines:** These are bacteriostatic agents that inhibit protein synthesis by binding to the **30S ribosomal subunit**, preventing the attachment of aminoacyl-tRNA to the A-site. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Resistance to Rifampicin occurs due to mutations in the **rpoB gene**, which alters the $\beta$-subunit of RNA polymerase. * **Metabolism:** It is a potent **microsomal enzyme inducer** (CYP450), leading to significant drug interactions (e.g., reducing the efficacy of oral contraceptives, warfarin, and HIV protease inhibitors). * **Side Effects:** A characteristic side effect is the **orange-red discoloration** of body fluids (urine, sweat, tears), which is harmless but requires patient counseling. * **Clinical Use:** It is a first-line drug for TB and Leprosy and is the drug of choice for **prophylaxis of Meningococcal and H. influenzae meningitis**.

Question 692: What is a known adverse effect of Zidovudine?

A. Neurotoxicity
B. Nephrotoxicity
C. Neutropenia (Correct Answer)
D. Ototoxicity

Explanation: **Explanation:** **Zidovudine (AZT)** is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in the management of HIV. The correct answer is **Neutropenia** because Zidovudine is notorious for causing significant **bone marrow suppression**. 1. **Why Neutropenia is Correct:** Zidovudine inhibits DNA polymerase-gamma in host cells, leading to mitochondrial toxicity and interference with hematopoiesis. This manifests clinically as **macrocytic anemia** and **neutropenia**. These side effects are dose-dependent and are the most common reasons for discontinuing or switching the drug. 2. **Why Other Options are Incorrect:** * **Neurotoxicity:** While some NRTIs (like Stavudine or Didanosine) cause peripheral neuropathy, Zidovudine is more commonly associated with **myopathy** (skeletal muscle wasting) rather than neurotoxicity. * **Nephrotoxicity:** This is a classic side effect of **Tenofovir** (another NRTI) or Amphotericin B, but not Zidovudine. * **Ototoxicity:** This is characteristic of Aminoglycosides (e.g., Gentamicin) or Loop diuretics, not antiretroviral therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Zidovudine:** Remember the **"3 Ms"**: **M**yelosuppression (Anemia/Neutropenia), **M**yopathy, and **M**acrocytosis (increased MCV is an early marker of compliance). * **Drug of Choice:** Zidovudine is the preferred agent for preventing **vertical transmission** (mother-to-child) of HIV during pregnancy and labor. * **Drug Interaction:** Avoid co-administration with **Ganciclovir**, as both cause additive bone marrow toxicity.

Question 693: What is the drug of choice for chemoprophylaxis in contacts of a patient with pneumonic plague?

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

Explanation: **Explanation:** **Pneumonic Plague**, caused by *Yersinia pestis*, is a highly contagious and fatal form of the disease transmitted via respiratory droplets. Due to its rapid progression and high mortality rate, immediate chemoprophylaxis is mandatory for all close contacts. **1. Why Tetracycline is the Correct Answer:** **Tetracyclines** (specifically **Doxycycline** or Tetracycline hydrochloride) are the drugs of choice for the chemoprophylaxis of plague. They are highly effective against *Y. pestis* and provide reliable protection when administered within the incubation period. In clinical practice, Doxycycline is often preferred due to its superior pharmacokinetic profile (longer half-life and better compliance). **2. Why the Other Options are Incorrect:** * **A. Penicillin:** *Yersinia pestis* is naturally resistant to penicillins; they have no role in the treatment or prophylaxis of plague. * **B. Rifampicin:** While used for prophylaxis in Meningococcal meningitis and H. influenzae, it is not effective against the plague bacillus. * **C. Erythromycin:** Macrolides are generally not considered first-line agents for plague and are less effective than tetracyclines or aminoglycosides. **3. Clinical Pearls for NEET-PG:** * **Drug of Choice for Treatment:** For active cases of Bubonic or Pneumonic plague, **Streptomycin** (an Aminoglycoside) is the traditional drug of choice. Gentamicin is a common alternative. * **Alternative Prophylaxis:** If tetracyclines are contraindicated (e.g., in young children or pregnancy), **Sulfonamides** (like Co-trimoxazole) can be used, though they are less effective. * **Vector:** The primary vector for plague is the rat flea (*Xenopsylla cheopis*). * **Safety Note:** In a bioterrorism scenario, Ciprofloxacin is also recommended by the CDC for both treatment and prophylaxis.

Question 694: What is the drug of choice for pertussis?

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

Explanation: **Explanation:** **1. Why Erythromycin is the Correct Answer:** Pertussis (Whooping Cough) is caused by the Gram-negative coccobacillus *Bordetella pertussis*. **Macrolides** are the drugs of choice for both treatment and post-exposure prophylaxis. While newer macrolides like Azithromycin and Clarithromycin are often preferred in clinical practice due to better tolerability and shorter courses, **Erythromycin** remains the classic "textbook" drug of choice and the standard answer for competitive exams like NEET-PG. Macrolides work by inhibiting protein synthesis (binding to the 50S ribosomal subunit) and are most effective when started during the catarrhal stage to reduce the period of communicability. **2. Why Other Options are Incorrect:** * **Penicillin:** *B. pertussis* is inherently resistant to penicillins in vivo. Furthermore, Penicillin G/V lack the necessary spectrum to cover atypical respiratory pathogens. * **Doxycycline:** While tetracyclines have broad-spectrum activity, they are not first-line for pertussis and are generally avoided in young children (the primary demographic for pertussis) due to the risk of bone growth inhibition and tooth discoloration. * **Ciprofloxacin:** Fluoroquinolones are not indicated for pertussis; they are typically reserved for other respiratory infections or urinary tract infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alternative:** If a patient is intolerant to macrolides, **Trimethoprim-Sulfamethoxazole (TMP-SMX)** is the preferred alternative. * **Infant Caution:** In neonates (<1 month), **Azithromycin** is preferred over Erythromycin because Erythromycin is associated with an increased risk of **Infantile Hypertrophic Pyloric Stenosis (IHPS)**. * **Clinical Stages:** Treatment is most effective in the **Catarrhal stage**; once the **Paroxysmal stage** (whooping cough) begins, antibiotics limit spread but have little effect on the clinical course.

Question 695: Cilastatin is given along with which of the following agents?

A. Imipenem (Correct Answer)
B. Amoxicillin
C. Erythromycin
D. Ampicillin

Explanation: ### Explanation **Correct Option: A (Imipenem)** **The Mechanism:** Imipenem is a broad-spectrum carbapenem antibiotic. When administered alone, it is rapidly inactivated in the kidneys by the enzyme **Dehydropeptidase-I (DHP-I)**, located in the brush border of the proximal renal tubule. This metabolism leads to two major issues: 1. **Reduced Efficacy:** Lower serum levels of the active drug. 2. **Nephrotoxicity:** The metabolites produced are potentially toxic to the renal tubules. **Cilastatin** is a specific, reversible **DHP-I inhibitor**. It has no antibacterial activity of its own but is co-administered with Imipenem in a 1:1 ratio to prevent its hydrolysis, thereby increasing its plasma concentration and reducing nephrotoxicity. --- ### Why Other Options are Incorrect: * **B & D (Amoxicillin/Ampicillin):** These are aminopenicillins. They are often combined with **Beta-lactamase inhibitors** (like Clavulanic acid or Sulbactam) to prevent degradation by bacterial enzymes, not DHP-I. * **C (Erythromycin):** This is a macrolide antibiotic. It does not undergo renal metabolism by DHP-I and therefore does not require Cilastatin. --- ### High-Yield Clinical Pearls for NEET-PG: * **"The Carbapenem Rule":** Newer carbapenems like **Meropenem, Ertapenem, and Doripenem** are resistant to DHP-I degradation and do **not** require Cilastatin. * **Adverse Effects:** Imipenem-Cilastatin is known to lower the seizure threshold (neurotoxicity), especially in patients with renal failure. * **Spectrum:** Imipenem is a "reserve drug" used for serious mixed aerobic/anaerobic infections and ESBL-producing organisms. * **Mnemonic:** "Keep it **Clean** with **Cila**statin" (Cilastatin keeps the Imipenem active and the kidneys clean).

Question 696: Amphotericin B is indicated for which type of infection?

A. Fungal (Correct Answer)
B. Viral
C. Bacterial
D. Rickettsial

Explanation: **Explanation:** **Amphotericin B** is a potent polyene macrolide antibiotic used primarily for systemic life-threatening fungal infections. Its mechanism of action involves binding to **ergosterol**, a vital component of the fungal cell membrane. This binding creates pores or channels in the membrane, leading to the leakage of intracellular ions (like potassium) and molecules, ultimately resulting in cell death (fungicidal action). **Analysis of Options:** * **Option A (Correct):** It is the "gold standard" for most systemic mycoses, including Candidiasis, Cryptococcosis, Histoplasmosis, and Mucormycosis. * **Option B (Incorrect):** Viral infections are treated with antivirals that target viral replication cycles (e.g., DNA polymerase inhibitors like Acyclovir). Amphotericin B has no activity against viruses. * **Option C (Incorrect):** While derived from *Streptomyces nodosus* (a bacterium), it does not target bacterial cell walls (peptidoglycan) or ribosomes. * **Option D (Incorrect):** Rickettsial infections are intracellular bacterial infections typically treated with Tetracyclines (Doxycycline) or Chloramphenicol. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Resistance:** Occurs via a decrease in the ergosterol content of the fungal membrane. * **Adverse Effects:** 1. **Infusion-related:** Fever, chills ("Shake and Bake" phenomenon). 2. **Dose-limiting toxicity:** Nephrotoxicity (causes distal renal tubular acidosis and hypokalemia). 3. **Anemia:** Due to decreased erythropoietin production. * **Liposomal Amphotericin B:** Developed to reduce nephrotoxicity by targeting the drug specifically to fungal cells and sparing mammalian membranes.

Question 697: Which of the following antimicrobial agents is NOT used in the treatment of Klebsiella infections?

A. Ceftazidime
B. Ciprofloxacin
C. Gentamicin
D. Erythromycin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Erythromycin**. **1. Why Erythromycin is the correct answer:** *Klebsiella pneumoniae* is a **Gram-negative**, aerobic bacillus. Erythromycin is a **Macrolide** antibiotic that primarily targets Gram-positive bacteria and "atypical" pathogens (like *Mycoplasma* or *Legionella*). Gram-negative organisms like *Klebsiella* possess an outer membrane that is relatively impermeable to large, hydrophobic molecules like Erythromycin, making them **innately resistant** to this drug. **2. Why the other options are incorrect:** * **Ceftazidime (Option A):** A 3rd-generation Cephalosporin with excellent activity against Gram-negative bacteria, including *Klebsiella*. It is often used for serious infections, though resistance via Extended-Spectrum Beta-Lactamases (ESBL) is rising. * **Ciprofloxacin (Option B):** A Fluoroquinolone that inhibits DNA gyrase. It is commonly used for urinary tract infections and pneumonia caused by *Klebsiella*. * **Gentamicin (Option C):** An Aminoglycoside that inhibits the 30S ribosomal subunit. It is highly effective against aerobic Gram-negative bacilli and is often used in combination therapy for *Klebsiella* sepsis. **3. NEET-PG High-Yield Pearls:** * **Klebsiella Characteristics:** Non-motile, encapsulated (mucoid colonies), and a common cause of "currant jelly sputum" pneumonia in alcoholics and diabetics. * **Drug of Choice:** For ESBL-producing *Klebsiella*, **Carbapenems** (e.g., Meropenem) are the drugs of choice. * **Emerging Threat:** KPC (*Klebsiella pneumoniae* carbapenemase) producing strains require treatment with **Colistin, Tigecycline,** or newer agents like **Ceftazidime-Avibactam**.

Question 698: Which antiretroviral drug can bind to both CCR2 and CCR5?

A. Cenicriviroc (Correct Answer)
B. Vicriviroc
C. Ibalizumab
D. Bevirimat

Explanation: **Explanation:** **Cenicriviroc** is a novel, investigational entry inhibitor that acts as a **dual antagonist** of both **CCR2 and CCR5** receptors. While CCR5 is a well-known co-receptor for HIV-1 entry into CD4+ T cells, CCR2 is primarily involved in monocyte recruitment and inflammation. By blocking both, Cenicriviroc not only inhibits viral entry but also exerts potent anti-inflammatory and anti-fibrotic effects, making it a significant drug of interest for treating Non-Alcoholic Steatohepatitis (NASH) alongside HIV. **Analysis of Incorrect Options:** * **Vicriviroc:** This is a selective **CCR5 antagonist** (similar to Maraviroc). It does not have significant activity against the CCR2 receptor. * **Ibalizumab:** This is a **post-attachment inhibitor**. It is a humanized monoclonal antibody that binds to domain 2 of the **CD4 receptor**, preventing the conformational changes required for viral entry without causing immunosuppression. * **Bevirimat:** This belongs to a class called **Maturation Inhibitors**. It inhibits the final step of Gag processing, leading to the production of defective, non-infectious virions. **High-Yield Pearls for NEET-PG:** * **Maraviroc:** The only FDA-approved CCR5 antagonist; requires a **Trofile assay** before use to ensure the patient has R5-tropic virus (it is ineffective against X4-tropic or dual-tropic virus). * **Enfuvirtide:** A fusion inhibitor that binds to the **gp41** subunit of the viral envelope. * **CCR5 Mutation:** Individuals with a homozygous **CCR5-Δ32 mutation** are naturally resistant to HIV-1 infection.

Question 699: All of the following drugs are used in the management of HIV/AIDS except:

A. Fanciclovir (Correct Answer)
B. Ritonavir
C. Stavudine
D. Indinavir

Explanation: **Explanation:** The correct answer is **Famciclovir** (Option A). **1. Why Famciclovir is the correct answer:** Famciclovir is a prodrug of penciclovir, which belongs to the class of **Anti-herpes drugs**. It acts by inhibiting viral DNA polymerase. It is specifically used for infections caused by the Herpes Simplex Virus (HSV-1, HSV-2) and Varicella-Zoster Virus (VZV/Shingles). It has **no activity against HIV**, as it does not inhibit Reverse Transcriptase or Protease enzymes. **2. Analysis of incorrect options:** * **Ritonavir (Option B):** This is a **Protease Inhibitor (PI)**. In modern Highly Active Antiretroviral Therapy (HAART), it is primarily used in low doses as a "pharmacokinetic enhancer" (booster) because it inhibits the CYP3A4 enzyme, increasing the plasma levels of other PIs. * **Stavudine (Option C):** This is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)**, a thymidine analogue. While its use has declined due to mitochondrial toxicity (lactic acidosis and lipodystrophy), it is a classic anti-HIV medication. * **Indinavir (Option D):** This is another **Protease Inhibitor (PI)** used in the management of HIV-1 and HIV-2. A notable side effect is nephrolithiasis (crystalluria). **3. NEET-PG High-Yield Pearls:** * **HAART Regimen:** Usually consists of 2 NRTIs + 1 NNRTI or 1 PI or 1 Integrase Inhibitor. * **Drug of Choice for HSV:** Acyclovir/Famciclovir. * **Mnemonic for PIs:** All Protease Inhibitors end in the suffix **"-navir"** (e.g., Ritonavir, Indinavir, Lopinavir). * **Zidovudine (AZT):** The first NRTI; key side effect is bone marrow suppression (anemia/neutropenia).

Question 700: Which drug can be safely used in the first trimester of pregnancy for the treatment of malaria?

A. Quinine (Correct Answer)
B. Amodiaquine
C. Artemether
D. Mefloquine

Explanation: The treatment of malaria in pregnancy is a high-yield topic for NEET-PG, as drug safety profiles change depending on the trimester. **1. Why Quinine is Correct:** According to the WHO and National Vector Borne Disease Control Programme (NVBDCP) guidelines, **Quinine + Clindamycin** is the drug of choice for the treatment of uncomplicated malaria (both *P. falciparum* and *P. vivax*) during the **first trimester** of pregnancy. Quinine has a long-standing safety record in early pregnancy and is not associated with teratogenicity, although it may cause hypoglycemia in the mother. **2. Analysis of Incorrect Options:** * **Artemether (Option C):** Artemisinin-based Combination Therapy (ACT) is the drug of choice for the **second and third trimesters**. However, in the first trimester, ACT is generally avoided unless Quinine is unavailable or the case is one of severe malaria, due to limited safety data regarding potential embryotoxicity [1]. * **Amodiaquine (Option B):** While used in some ACT formulations (e.g., Artesunate + Amodiaquine), it is not used as monotherapy and is not the primary recommendation for the first trimester. * **Mefloquine (Option D):** It is used for prophylaxis in pregnancy but is not the first-line treatment for acute malaria in the first trimester [1]. **3. Clinical Pearls for NEET-PG:** * **Severe Malaria in Pregnancy:** Regardless of the trimester, **IV Artesunate** is the drug of choice because the benefits of saving the mother's life outweigh the theoretical risks to the fetus. * **Vivax Malaria:** Chloroquine remains safe in all trimesters. However, **Primaquine is strictly contraindicated** throughout pregnancy due to the risk of hemolysis in the fetus [1]. * **Prophylaxis:** Chloroquine is safe for prophylaxis throughout pregnancy in sensitive areas.

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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