Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1131: Which of the following statements is NOT true regarding the use of silver sulfadiazine?

A. It is available in a 1% concentration.
B. It is commonly used for the treatment of burns.
C. It is considered safer for pregnant women and infants. (Correct Answer)
D. It acts against Staphylococcus aureus.

Explanation: ### Explanation **Silver Sulfadiazine (SSD)** is a topical sulfonamide widely used in burn management. The correct answer is **Option C** because it is **not** safe for pregnant women near term or for infants under two months of age. #### 1. Why Option C is the correct (False) statement: Sulfonamides, including silver sulfadiazine, compete with bilirubin for binding sites on serum albumin. In neonates and fetuses (near term), this leads to increased levels of free unconjugated bilirubin, which can cross the blood-brain barrier and cause **kernicterus** (bilirubin encephalopathy). Therefore, its use is contraindicated in pregnancy at term, during breastfeeding, and in infants <2 months old. #### 2. Analysis of Incorrect (True) Options: * **Option A:** SSD is standardly formulated as a **1% water-miscible cream**. * **Option B:** It is the **gold standard** topical agent for preventing and treating infections in second- and third-degree **burns**. * **Option D:** It has a broad spectrum of activity, covering both Gram-positive organisms like ***Staphylococcus aureus*** and Gram-negative organisms like *Pseudomonas aeruginosa*. #### 3. NEET-PG High-Yield Clinical Pearls: * **Mechanism:** It acts on the cell wall and cell membrane; unlike other sulfonamides, its action is primarily due to the slow release of **silver ions**, which are bactericidal. * **Advantage over Mafenide Acetate:** SSD does not inhibit carbonic anhydrase; therefore, it does **not** cause metabolic acidosis (a common side effect of mafenide). * **Side Effects:** Watch for transient **leukopenia** (usually resolves spontaneously) and skin discoloration. * **Limitation:** It has poor eschar penetration compared to mafenide acetate.

Question 1132: What is the drug of choice for Mycoplasma pneumoniae infection?

A. Gentamicin
B. Amoxicillin
C. Azithromycin (Correct Answer)
D. Cefotaxime

Explanation: **Explanation:** **1. Why Azithromycin is the Correct Answer:** *Mycoplasma pneumoniae* is an "atypical" bacterium that **lacks a peptidoglycan cell wall**. Therefore, it is inherently resistant to all beta-lactam antibiotics. The drug of choice for Mycoplasma is a **Macrolide**, such as **Azithromycin** (or Erythromycin/Clarithromycin). * **Mechanism:** Macrolides inhibit bacterial protein synthesis by binding to the **50S ribosomal subunit**. * **Clinical Efficacy:** Azithromycin is preferred due to its superior tissue penetration, long half-life (allowing once-daily dosing), and better tolerability compared to Erythromycin. **2. Why the Other Options are Incorrect:** * **Amoxicillin (Penicillin) & Cefotaxime (3rd Gen Cephalosporin):** These are Beta-lactams that act by inhibiting cell wall synthesis. Since *Mycoplasma* lacks a cell wall, these drugs are completely ineffective. * **Gentamicin (Aminoglycoside):** While it inhibits protein synthesis (30S subunit), it is primarily used for aerobic Gram-negative bacilli and is not the standard treatment for atypical pneumonia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Atypical Pneumonia:** Often presents as "Walking Pneumonia" (clinical symptoms are milder than radiological findings). * **Cold Agglutinins:** Infection with *M. pneumoniae* is associated with the development of cold agglutinin antibodies (IgM), which can lead to autoimmune hemolytic anemia. * **Alternative Drugs:** Tetracyclines (e.g., **Doxycycline**) and Fluoroquinolones (e.g., **Levofloxacin**) are also effective against *Mycoplasma*. * **Key Feature:** *Mycoplasma* is the smallest free-living organism and its cell membrane contains **sterols** (unlike other bacteria).

Question 1133: Which drug is used to treat Cryptococcal meningitis?

A. Pentostatin
B. Amphotericin B (Correct Answer)
C. Zidovudine
D. Clotrimazole

Explanation: **Explanation:** **Amphotericin B** is the gold-standard treatment for **Cryptococcal meningitis**, typically used in the induction phase. It is a polyene antifungal that binds to ergosterol in the fungal cell membrane, creating pores that lead to cell death (fungicidal). For Cryptococcal meningitis, it is administered intravenously, often in combination with **Flucytosine**, followed by a maintenance phase of Fluconazole. **Analysis of Incorrect Options:** * **Pentostatin (Option A):** A purine analog used as a chemotherapy agent, primarily for Hairy Cell Leukemia. It inhibits adenosine deaminase. * **Zidovudine (Option C):** A Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in the treatment of HIV/AIDS. While Cryptococcal meningitis is an AIDS-defining illness, Zidovudine treats the virus, not the fungal infection. * **Clotrimazole (Option D):** A topical imidazole antifungal used for superficial infections like oral thrush, vaginal candidiasis, or tinea. It is not used for systemic or CNS infections due to poor penetration and toxicity if given systemically. **High-Yield Clinical Pearls for NEET-PG:** * **Liposomal Amphotericin B** is preferred over the conventional deoxycholate form to reduce the risk of **nephrotoxicity** (the most common dose-limiting side effect). * **Infusion-related reactions:** "Shake and Bake" (fever and chills) is a classic side effect of Amphotericin B. * **Electrolyte Imbalance:** It frequently causes hypokalemia and hypomagnesemia. * **DOC Summary:** Amphotericin B is also the drug of choice for Mucormycosis and Visceral Leishmaniasis (Kala-azar).

Question 1134: Which of the following is not a bactericidal agent?

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

Explanation: **Explanation:** In the treatment of Tuberculosis, anti-tubercular drugs (ATDs) are classified based on their mechanism of action as either **bactericidal** (kill the bacteria) or **bacteriostatic** (inhibit growth/multiplication). **Why Ethambutol is the correct answer:** Ethambutol is the only primary (first-line) anti-tubercular drug that is **bacteriostatic**. It works by inhibiting the enzyme **arabinosyl transferase**, which interferes with the synthesis of arabinogalactan, a critical component of the mycobacterial cell wall. By halting cell wall assembly rather than causing immediate cell death, it prevents the multiplication of the bacilli. **Analysis of incorrect options:** * **Rifampicin:** A potent bactericidal drug. It inhibits DNA-dependent RNA polymerase, preventing transcription. It is effective against both rapidly dividing and dormant (persister) bacilli. * **INH (Isoniazid):** Highly bactericidal against rapidly dividing mycobacteria. It inhibits the synthesis of mycolic acids (via the InhA target). It is the most effective drug for rendering a patient non-infectious quickly. * **Pyrazinamide:** Bactericidal in an acidic medium (specifically within macrophages). It is unique for its ability to kill slow-growing intracellular bacilli, making it essential for shortening the duration of therapy. **NEET-PG High-Yield Pearls:** * **Mnemonic for Bacteriostatic drugs:** "**E**thambutol is **E**mpty of killing power" (Bacteriostatic). * **Visual Side Effect:** Ethambutol is notorious for causing **Retrobulbar Neuritis**. Patients must be monitored for changes in visual acuity and **red-green color blindness**. * **Safe in Pregnancy:** All first-line ATDs (HRZE) are generally considered safe in pregnancy, though Streptomycin (an aminoglycoside) is avoided due to ototoxicity. * **Pyrazinamide** is the most common cause of drug-induced hyperuricemia (gout) among ATDs.

Question 1135: Combination of sulfamethoxazole and trimethoprim is synergistic due to which of the following mechanisms?

A. Enhanced entry into bacteria
B. Sequential blockade in folate metabolism (Correct Answer)
C. Inhibition of drug inactivation enzymes
D. Inhibition of drug efflux

Explanation: The combination of **Sulfamethoxazole and Trimethoprim** (known as Co-trimoxazole) is a classic example of **pharmacodynamic synergy** through **sequential blockade** of a metabolic pathway. ### Why Option B is Correct Bacteria cannot absorb preformed folic acid and must synthesize it de novo for DNA synthesis. The combination inhibits two consecutive steps in this pathway: 1. **Sulfamethoxazole:** A structural analog of PABA, it competitively inhibits the enzyme **Dihydropteroate Synthase**. 2. **Trimethoprim:** Inhibits the enzyme **Dihydrofolate Reductase (DHFR)**, preventing the reduction of dihydrofolate to tetrahydrofolate (the active form). By attacking two different points in the same biosynthetic chain, the combination becomes **bactericidal**, whereas the individual drugs are usually only bacteriostatic. This also reduces the development of bacterial resistance. ### Why Other Options are Incorrect * **Option A:** This mechanism describes the synergy between **Cell Wall Inhibitors (e.g., Penicillins)** and **Aminoglycosides**, where the cell wall damage allows better entry of the aminoglycoside. * **Option C:** This describes the mechanism of **Beta-lactamase inhibitors** (e.g., Clavulanic acid) when combined with Penicillins. * **Option D:** While inhibiting efflux pumps is a strategy to reverse resistance, it is not the mechanism of action for Co-trimoxazole. ### High-Yield NEET-PG Pearls * **Ratio:** The drugs are administered in a **1:5 ratio** (80mg Trimethoprim : 400mg Sulfamethoxazole) to achieve a **1:20 ratio** in the plasma, which is the optimal synergistic concentration. * **Drug of Choice (DOC):** Co-trimoxazole remains the DOC for *Pneumocystis jirovecii* pneumonia and *Nocardia* infections. * **Adverse Effects:** Watch for **Stevens-Johnson Syndrome (SJS)** due to the sulfonamide component and **megaloblastic anemia** due to folate deficiency.

Question 1136: Which of the following is a side effect of streptomycin?

A. Phototoxicity
B. Hepatotoxicity
C. Ototoxicity (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Streptomycin**, an aminoglycoside antibiotic, is primarily used in the treatment of tuberculosis and certain gram-negative infections. Its most significant and dose-limiting side effects are **Ototoxicity** and **Nephrotoxicity**. 1. **Why Ototoxicity is correct:** Aminoglycosides like streptomycin accumulate in the endolymph and perilymph of the inner ear. They cause irreversible damage to the sensory hair cells. Specifically, streptomycin is more **vestibulotoxic** than cochleotoxic, leading to symptoms like vertigo, ataxia, and loss of balance. 2. **Why other options are incorrect:** * **Phototoxicity:** This is a classic side effect of Tetracyclines, Fluoroquinolones, and Sulfonamides, but not aminoglycosides. * **Hepatotoxicity:** Streptomycin is excreted unchanged by the kidneys and does not undergo significant hepatic metabolism; therefore, it is not typically associated with liver injury (unlike other anti-TB drugs like Isoniazid, Rifampicin, and Pyrazinamide). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits bacterial protein synthesis by binding to the **30S ribosomal subunit**, causing mRNA misreading. * **Pregnancy:** Streptomycin is **Contraindicated** in pregnancy as it can cause fetal ototoxicity (8th cranial nerve damage). * **Neuromuscular Blockade:** Aminoglycosides can cause a curare-like effect by inhibiting acetylcholine release; this is reversed by Calcium gluconate or Neostigmine. * **Mnemonic for Aminoglycoside Toxicity:** **"N.O.W."** (**N**ephrotoxicity, **O**totoxicity, **W**eakness/Neuromuscular blockade).

Question 1137: Which penicillin is acid-labile?

A. Methicillin (Correct Answer)
B. Ampicillin
C. Amoxicillin
D. Cloxacillin

Explanation: **Explanation:** The stability of penicillins in the presence of gastric acid determines whether they can be administered orally. **Acid-labile** drugs are destroyed by gastric acid and must be given parenterally (IV/IM), whereas **acid-stable** drugs can be given orally. **Why Methicillin is the correct answer:** Methicillin is a narrow-spectrum, penicillinase-resistant penicillin. It is highly **acid-labile**, meaning it is rapidly degraded by the low pH of the stomach. Consequently, it cannot be administered orally and was historically given only by injection. Clinically, it is no longer used due to nephrotoxicity (interstitial nephritis), but it remains the laboratory standard for defining MRSA (Methicillin-Resistant *Staphylococcus aureus*). **Analysis of Incorrect Options:** * **Ampicillin:** An extended-spectrum penicillin that is acid-stable and can be given orally, though its absorption is decreased by food. * **Amoxicillin:** An extended-spectrum penicillin that is highly acid-stable with better oral bioavailability than ampicillin; its absorption is not affected by food. * **Cloxacillin:** An antistaphylococcal (penicillinase-resistant) penicillin that is acid-stable and can be administered orally, unlike methicillin. **NEET-PG High-Yield Pearls:** * **Acid-Labile Penicillins (Parenteral only):** Penicillin G (Natural), Methicillin, Carbenicillin, Piperacillin, Ticarcillin. * **Acid-Stable Penicillins (Oral):** Penicillin V, Amoxicillin, Ampicillin, Cloxacillin, Dicloxacillin. * **Drug of Choice for Syphilis:** Penicillin G (Benzathine penicillin). * **Interstitial Nephritis:** A classic side effect associated with Methicillin in board exams.

Question 1138: Which antiviral drug exhibits dual activity against HIV and HBV?

A. Enfuvirtide
B. Emtricitabine (Correct Answer)
C. Abacavir
D. Entecavir

Explanation: ### Explanation **Correct Answer: B. Emtricitabine** **Mechanism and Rationale:** Emtricitabine is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)**. It is a cytosine analogue that inhibits the HIV reverse transcriptase enzyme. Crucially, the HBV DNA polymerase (which also functions as a reverse transcriptase) shares structural similarities with the HIV enzyme. Therefore, Emtricitabine effectively inhibits both viruses. This dual activity makes it a cornerstone in treating patients co-infected with HIV and HBV. **Analysis of Incorrect Options:** * **A. Enfuvirtide:** This is a **fusion inhibitor** that binds to the gp41 subunit of the HIV viral envelope. It prevents the virus from entering the host cell but has no activity against HBV. * **C. Abacavir:** While this is an NRTI used for HIV, it is a guanosine analogue that **lacks significant activity against HBV**. It is also uniquely associated with HLA-B*5701 hypersensitivity reactions. * **D. Entecavir:** This is a potent nucleoside analogue used specifically for **HBV**. While it inhibits HBV DNA polymerase, it does not have clinically significant activity against HIV at standard doses and is not used for HIV treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Dual-Activity Drugs:** The three main drugs active against both HIV and HBV are **Tenofovir (TDF/TAF), Emtricitabine (FTC), and Lamivudine (3TC).** * **Treatment Strategy:** In HIV/HBV co-infection, the ART regimen must include two drugs active against HBV (usually Tenofovir + Emtricitabine/Lamivudine) to prevent the development of HBV resistance. * **Withdrawal Flare:** Discontinuing these drugs in a co-infected patient can lead to a severe, acute exacerbation of Hepatitis B. * **Mnemonic:** Remember **"LET"** for dual activity: **L**amivudine, **E**mtricitabine, **T**enofovir.

Question 1139: Despite its "age," penicillin G remains the drug of choice in the treatment of infections caused by which of the following microorganisms?

A. Bacteroides fragilis
B. Treponema pallidum (Correct Answer)
C. Haemophilus influenzae
D. Escherichia coli

Explanation: ### Explanation **Correct Answer: B. Treponema pallidum** Penicillin G (Benzylpenicillin) remains the **gold standard** and drug of choice for **Syphilis**, caused by the spirochete *Treponema pallidum*. Despite decades of use, the organism has remarkably failed to develop significant resistance to penicillin. For primary, secondary, or early latent syphilis, a single intramuscular dose of **Benzathine Penicillin G** is curative. For neurosyphilis, intravenous Aqueous Penicillin G is required to achieve therapeutic concentrations in the CSF. **Why the other options are incorrect:** * **A. Bacteroides fragilis:** This is an anaerobic gram-negative rod that produces **beta-lactamase**. It is inherently resistant to Penicillin G. The drugs of choice are Metronidazole, Carbapenems, or Beta-lactam/Beta-lactamase inhibitor combinations (e.g., Piperacillin-Tazobactam). * **C. Haemophilus influenzae:** Most strains produce beta-lactamases or have altered penicillin-binding proteins (PBPs). Third-generation cephalosporins (Ceftriaxone) or Amoxicillin-Clavulanate are preferred. * **D. Escherichia coli:** As a gram-negative aerobe, *E. coli* has an outer membrane that limits Penicillin G penetration, and most strains produce penicillinases. It is typically treated with Cephalosporins, Fluoroquinolones, or Aminoglycosides. **NEET-PG High-Yield Pearls:** * **Jarisch-Herxheimer Reaction:** A classic systemic reaction (fever, chills, hypotension) occurring within hours of starting penicillin for syphilis due to the release of endotoxins from dying spirochetes. * **Repository Penicillins:** Benzathine Penicillin G has the longest duration of action (up to 3-4 weeks) and is also the drug of choice for **Rheumatic Fever prophylaxis**. * **Other Indications:** Penicillin G remains the drug of choice for *Actinomyces israelii*, *Clostridium perfringens* (Gas gangrene), and *Neisseria meningitidis* (if susceptible).

Question 1140: Which anti-tubercular drug is associated with psychosis?

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

Explanation: **Explanation:** **Isoniazid (INH)** is the correct answer because it interferes with the metabolism of Vitamin B6 (Pyridoxine). INH inhibits the enzyme *pyridoxine phosphokinase*, preventing the conversion of pyridoxine to its active form, pyridoxal phosphate. This deficiency leads to decreased synthesis of neurotransmitters like GABA. Clinically, this manifests as **peripheral neuropathy** (most common) and **neuropsychiatric side effects**, including psychosis, irritability, and seizures. **Analysis of Incorrect Options:** * **Rifampicin:** Primarily associated with hepatotoxicity and the harmless orange-red discoloration of body fluids (urine, sweat, tears). It is a potent inducer of cytochrome P450 enzymes. * **Ethambutol:** Its hallmark side effect is **optic neuritis**, resulting in decreased visual acuity and red-green color blindness. It is generally not associated with CNS or psychiatric symptoms. * **Streptomycin:** An aminoglycoside that causes **ototoxicity** (vestibular and auditory nerve damage) and nephrotoxicity. It does not cross the blood-brain barrier effectively to cause psychosis. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** To prevent INH-induced neuropathy and psychosis, **Pyridoxine (10–50 mg/day)** is co-administered, especially in high-risk groups (alcoholics, diabetics, and pregnant women). * **Metabolism:** INH is metabolized by **Acetylation**. "Slow acetylators" are at a higher risk of peripheral neuropathy and psychosis. * **Other Psychotropic TB Drugs:** While not in the options, **Cycloserine** is another potent cause of "TB-induced madness" or psychosis. Ethionamide may also cause psychiatric disturbances.

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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