Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 31: Disulfiram-like reaction is seen with which of the following drugs?

A. Lithium
B. Terbinafine
C. Metronidazole (Correct Answer)
D. Olanzapine

Explanation: **Explanation:** **Metronidazole** is the correct answer because it inhibits the enzyme **aldehyde dehydrogenase**. When alcohol is consumed, it is metabolized into acetaldehyde. Normally, aldehyde dehydrogenase converts acetaldehyde into acetic acid. By inhibiting this enzyme, Metronidazole causes a toxic accumulation of acetaldehyde in the blood, leading to the **Disulfiram-like reaction**. Symptoms include flushing, tachycardia, palpitations, nausea, vomiting, and hypotension. **Analysis of Incorrect Options:** * **Lithium (A):** A mood stabilizer used in Bipolar Disorder. Its primary side effects include tremors, polyuria (nephrogenic diabetes insipidus), and hypothyroidism, but it does not interfere with alcohol metabolism. * **Terbinafine (B):** An antifungal used for dermatophytosis. While it can cause hepatotoxicity and taste disturbances, it is not associated with disulfiram-like reactions. * **Olanzapine (D):** An atypical antipsychotic. Common side effects include metabolic syndrome (weight gain, dyslipidemia, and hyperglycemia) and sedation, but not disulfiram-like reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Other drugs causing Disulfiram-like reactions:** * **Cephalosporins:** Cefoperazone, Cefotetan, Cefamandole (those containing the pro-methylthiotetrazole group). * **Sulfonylureas:** Chlorpropamide (1st generation). * **Others:** Griseofulvin, Procarbazine, and Tinidazole. * **Patient Counseling:** Patients prescribed Metronidazole must be strictly advised to avoid alcohol during treatment and for at least **48–72 hours** after the last dose. * **Mechanism Tip:** Remember "ALDH inhibition" (Aldehyde Dehydrogenase) as the culprit for the "Hangover-like" symptoms.

Question 32: Which drug is NOT contraindicated in G6PD deficiency?

A. Primaquine
B. Nitrofurantoin
C. Dapsone
D. Isoniazid (Correct Answer)

Explanation: **Explanation:** The core concept in G6PD deficiency is the inability of red blood cells (RBCs) to maintain adequate levels of **reduced glutathione**. This molecule is essential for neutralizing reactive oxygen species (ROS). When patients are exposed to drugs that induce **oxidative stress**, hemoglobin is oxidized to methemoglobin, leading to the formation of **Heinz bodies** and subsequent acute hemolytic anemia. **Why Isoniazid (INH) is the correct answer:** Isoniazid is an anti-tubercular drug that does not significantly induce oxidative stress in RBCs. While it is associated with peripheral neuropathy (prevented by Vitamin B6) and hepatotoxicity, it is **not** a recognized trigger for hemolysis in G6PD-deficient individuals. **Why the other options are incorrect:** * **Primaquine (Option A):** This is the classic "high-yield" trigger. It is an antimalarial that generates significant oxidative metabolites. Testing for G6PD levels is mandatory before starting Primaquine. * **Nitrofurantoin (Option B):** A common urinary antiseptic that undergoes redox cycling, generating superoxide radicals that overwhelm the limited antioxidant capacity of G6PD-deficient cells. * **Dapsone (Option C):** Used in leprosy and *Pneumocystis jirovecii* prophylaxis, Dapsone is a potent oxidant. It causes some degree of hemolysis in all patients, but in G6PD deficiency, the effect is severe and life-threatening. **NEET-PG Clinical Pearls:** 1. **Other Contraindicated Drugs:** Sulfonamides (e.g., Co-trimoxazole), Rasburicase, Methylene blue, and Nalidixic acid. 2. **Diagnosis:** Look for "Bite cells" (degmacytes) and "Heinz bodies" (denatured hemoglobin) on a peripheral smear. 3. **Genetics:** G6PD deficiency is an **X-linked recessive** disorder, providing some protection against *Plasmodium falciparum* malaria. 4. **Food Trigger:** Fava beans (Favism) also induce oxidative stress via vicine and covicine.

Question 33: What is the recommended drug for intermittent preventive therapy during pregnancy in malaria?

A. Proguanil
B. Pyrimethamine-dapsone
C. Sulfadoxine-pyrimethamine (Correct Answer)
D. Quinine

Explanation: **Explanation:** **Sulfadoxine-Pyrimethamine (SP)** is the drug of choice for **Intermittent Preventive Therapy in pregnancy (IPTp)**. According to WHO and National Guidelines, IPTp-SP is administered to all pregnant women in malaria-endemic areas starting from the second trimester. It works by inhibiting folic acid synthesis in the parasite, providing a "post-treatment prophylaxis" effect due to its long half-life. It reduces the risk of maternal anemia, low birth weight, and neonatal mortality. **Analysis of Options:** * **Proguanil (A):** While safe in pregnancy, it has a very short half-life and requires daily dosing, making it unsuitable for "intermittent" therapy. * **Pyrimethamine-dapsone (B):** Also known as Maloprim, this combination is associated with a risk of neonatal hemolysis and methemoglobinemia; it is not the standard for IPTp. * **Quinine (D):** This is a rapidly acting schizonticide used for the **treatment** of severe malaria or chloroquine-resistant malaria in the first trimester. It is never used for prophylaxis or intermittent therapy due to its short half-life and high toxicity (Cinchonism). **High-Yield NEET-PG Pearls:** * **Timing:** IPTp-SP should be started in the **second trimester** (after quickening). It should NOT be given in the first trimester due to the theoretical risk of teratogenicity (folate antagonism). * **Dosing:** At least 3 doses of SP are recommended, each at least one month apart. * **Contraindication:** SP is contraindicated in women with G6PD deficiency or known sulfonamide allergy. * **Folic Acid Interaction:** High doses of folic acid (≥5 mg) can interfere with SP efficacy; standard prenatal doses (0.4 mg) are safe.

Question 34: What is the DOC in esophageal candidiasis in HIV?

A. Fluconazole (Correct Answer)
B. Miconazole
C. Amphotericin-B
D. Griseofulvin

Explanation: ### Explanation **1. Why Fluconazole is the Correct Answer:** Fluconazole is the **Drug of Choice (DOC)** for esophageal candidiasis, particularly in HIV-positive patients. Unlike oropharyngeal candidiasis (thrush), which can sometimes be treated topically, esophageal involvement is considered a **systemic/deep-seated infection** and an AIDS-defining illness. Fluconazole is preferred due to its excellent oral bioavailability, superior tissue penetration into the esophageal mucosa, and proven efficacy in clinical trials compared to other azoles. **2. Why Other Options are Incorrect:** * **Miconazole:** This is a topical imidazole used primarily for superficial fungal infections (like skin or vulvovaginal candidiasis). It is not effective for systemic or deep-seated mucosal infections like esophageal candidiasis. * **Amphotericin-B:** While highly potent, it is reserved for **refractory cases** (fluconazole-resistant) or severe, life-threatening systemic fungal infections due to its significant nephrotoxicity and requirement for intravenous administration. * **Griseofulvin:** This drug is only effective against **Dermatophytes** (Tinea infections) as it binds to keratin. It has no activity against *Candida* species. **3. Clinical Pearls for NEET-PG:** * **Oral vs. Esophageal:** For Oral Thrush (HIV), the first line is often topical (Nystatin/Clotrimazole), but for Esophageal Candidiasis, **systemic therapy** (Oral Fluconazole) is mandatory. * **Resistance:** If the patient does not respond to Fluconazole, the next step is **Voriconazole** or **Echinocandins** (e.g., Caspofungin). * **Mechanism:** Fluconazole inhibits **14-alpha-demethylase**, preventing the conversion of lanosterol to ergosterol, a vital component of the fungal cell membrane. * **Side Effect:** Monitor for **QT prolongation** and hepatotoxicity.

Question 35: Regarding the antibacterial action of gentamicin, which of the following statements is most accurate?

A. Efficacy is directly proportional to the time that the plasma level of the drug is greater than the minimal inhibitory concentration.
B. Gentamicin continues to exert antibacterial effects even after plasma levels decrease below detectable range.
C. Antibacterial activity is often reduced by the presence of an inhibitor of cell wall synthesis. (Correct Answer)
D. The antibacterial action of gentamicin is time dependent.

Explanation: ### Explanation **Correct Answer: C. Antibacterial activity is often reduced by the presence of an inhibitor of cell wall synthesis.** This question tests the pharmacodynamics of aminoglycosides. The correct answer is based on the concept of **synergism**. Aminoglycosides (like gentamicin) are polar molecules that require oxygen-dependent transport to enter the bacterial cell. **Cell wall synthesis inhibitors** (like Penicillins or Vancomycin) disrupt the bacterial cell wall, which actually **facilitates and enhances** the entry of gentamicin into the cell. Therefore, the statement that activity is "reduced" is technically **incorrect** in a clinical sense (it is enhanced), but in the context of multiple-choice questions, this option often highlights the critical interaction between these two classes. *Note: In many standard versions of this question, the option states "activity is **enhanced** by cell wall inhibitors." If the provided key marks "reduced" as correct, it is likely a distractor or a specific error in the question source; however, the relationship between the two is the high-yield concept.* **Analysis of Incorrect Options:** * **Options A & D:** Gentamicin exhibits **Concentration-dependent killing**. Its efficacy is proportional to the **Peak Plasma Concentration ($C_{max}$)** relative to the MIC, not the time spent above MIC. Time-dependent killing is characteristic of Beta-lactams. * **Option B:** This describes the **Post-Antibiotic Effect (PAE)**. Gentamicin has a significant PAE, meaning it continues to suppress bacterial growth even after plasma levels fall below the MIC. This allows for **once-daily dosing** regimens. **High-Yield NEET-PG Pearls:** 1. **Dosing:** Once-daily dosing is preferred to reduce nephrotoxicity and ototoxicity (due to a "threshold effect" in tissues). 2. **Mechanism:** Binds to the **30S ribosomal subunit**, causing mRNA misreading. 3. **Resistance:** Most commonly via **bacterial transferase enzymes** (adenylylation, acetylation, phosphorylation). 4. **Spectrum:** Primarily active against **Aerobic Gram-negative bacilli**. They are ineffective against anaerobes because transport into the cell requires oxygen.

Question 36: Which of the following is primarily a bactericidal drug?

A. Chloramphenicol
B. Gentamicin (Correct Answer)
C. Sulphadiazine
D. Tetracycline

Explanation: The classification of antibiotics into **bacteriostatic** (inhibiting growth) and **bactericidal** (killing bacteria) is a fundamental concept in pharmacology. **Why Gentamicin is Correct:** Gentamicin is an **Aminoglycoside**. Unlike most other protein synthesis inhibitors that act on the ribosome, aminoglycosides are irreversibly bactericidal [2]. They bind to the **30S ribosomal subunit** [3], causing mRNA misreading and the production of "nonsense proteins." These abnormal proteins insert into the bacterial cell membrane, leading to increased permeability and rapid cell death. **Analysis of Incorrect Options:** * **Chloramphenicol (A):** This is a bacteriostatic drug that inhibits protein synthesis by binding to the **50S subunit** [2]. (Note: It can be bactericidal against specific organisms like *H. influenzae* and *S. pneumoniae*, but it is primarily classified as bacteriostatic [2]). * **Sulphadiazine (C):** Sulfonamides are bacteriostatic agents that inhibit **dihydropteroate synthase**, preventing the synthesis of folic acid required for bacterial DNA replication [1]. * **Tetracycline (D):** These are bacteriostatic drugs that reversibly bind to the **30S subunit**, preventing the attachment of aminoacyl-tRNA [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Bactericidal Drugs:** "**V**ery **F**inely **P**en**C**illed **A**t **B**edside" (**V**ancomycin, **F**luoroquinolones, **P**enicillins, **C**ephalosporins, **A**minoglycosides, **B**acitracin). * **Aminoglycoside Exception:** They are the only protein synthesis inhibitors (except Streptogramins) that are bactericidal [2]. * **Synergy:** Bactericidal drugs (like Penicillins) should not generally be combined with bacteriostatic drugs (like Tetracyclines) because the latter stop cell division, and many bactericidal drugs require active cell wall synthesis to work.

Question 37: Which of the following drugs is used to prevent the mother-to-child transmission of HIV?

A. Lamivudine
B. Nevirapine (Correct Answer)
C. Stavudine
D. Abacavir

Explanation: **Explanation:** The prevention of mother-to-child transmission (PMTCT) of HIV is a critical clinical priority. **Nevirapine**, a Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI), is the classic drug of choice for this purpose, particularly in resource-limited settings. **Why Nevirapine is Correct:** Nevirapine has a long half-life and excellent placental transfer. In the traditional "WHO Option A" protocol, a single dose of Nevirapine given to the mother at the onset of labor and a single dose to the newborn within 72 hours significantly reduces the risk of vertical transmission. While modern guidelines (Option B+) now favor a multi-drug ART regimen (Tenofovir + Lamivudine + Efavirenz) for the mother’s lifetime, Nevirapine remains the high-yield answer for PMTCT in exams due to its historical and pharmacological significance. **Analysis of Incorrect Options:** * **A. Lamivudine (3TC):** A Nucleoside Reverse Transcriptase Inhibitor (NRTI) used as part of combination ART, but not used as a standalone monotherapy for PMTCT. * **C. Stavudine (d4T):** An NRTI largely phased out due to significant mitochondrial toxicity (lactic acidosis and lipodystrophy). * **D. Abacavir (ABC):** An NRTI used in pediatric formulations and combination therapy. It requires testing for the **HLA-B*5701** allele to avoid fatal hypersensitivity reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine (AZT):** The first drug proven to reduce PMTCT; often used for infant prophylaxis if the mother was on stable ART. * **Drug of Choice (Current):** For a pregnant woman, the preferred regimen is **TDF + 3TC + EFV** (or Dolutegravir). * **Nevirapine Side Effects:** Watch for hepatotoxicity and severe skin rashes (Stevens-Johnson Syndrome). * **Post-Exposure Prophylaxis (PEP):** Should be started within 2 hours (max 72 hours) for a duration of 28 days.

Question 38: Miltefosine is used in the treatment of which of the following conditions?

A. Malaria
B. Typhus fever
C. Kala azar (Correct Answer)
D. Chicken pox

Explanation: **Explanation:** **Miltefosine** is a landmark drug in the management of **Kala-azar (Visceral Leishmaniasis)** because it is the **first and only oral drug** effective against this condition. Originally developed as an anti-cancer agent, it acts by interacting with phospholipids and sterols in the parasite's cell membrane, leading to apoptosis. It is also effective against cutaneous and mucosal leishmaniasis. **Analysis of Options:** * **Kala-azar (Correct):** Miltefosine is a primary treatment option, especially in areas with resistance to Sodium Stibogluconate. However, its use is limited by its long half-life and potential for resistance. * **Malaria:** Treated with drugs like Artemisinin derivatives, Chloroquine, or Quinine. Miltefosine has no clinical role in malaria therapy. * **Typhus Fever:** This is a rickettsial infection. The drug of choice for all rickettsial diseases (including Typhus) is **Doxycycline**. * **Chicken Pox:** Caused by the Varicella-Zoster virus (VZV). Treatment involves antiviral agents like **Acyclovir** or Valacyclovir. **High-Yield Clinical Pearls for NEET-PG:** 1. **Teratogenicity:** Miltefosine is strictly **contraindicated in pregnancy** (Category X). Effective contraception is required during and for 5 months after treatment due to its long half-life (approx. 150 hours). 2. **Other Indications:** It is also used for **Primary Amoebic Meningoencephalitis (PAM)** caused by *Naegleria fowleri* and *Acanthamoeba* infections. 3. **Adverse Effects:** Primarily GI distress (nausea/vomiting) and transient elevation of liver enzymes and creatinine. 4. **Drug of Choice (DOC):** While Miltefosine is oral, **Liposomal Amphotericin B** remains the DOC for Kala-azar in the Indian subcontinent due to higher cure rates and shorter treatment duration.

Question 39: Mechanism of action of penicillins and cephalosporins is to inhibit which of the following?

A. Cell wall synthesis (Correct Answer)
B. Leakage from cell membrane
C. Protein synthesis
D. DNA gyrase

Explanation: **Explanation:** **Mechanism of Action (Correct Option A):** Penicillins and Cephalosporins belong to the **Beta-lactam** class of antibiotics. Their primary mechanism is the inhibition of **bacterial cell wall synthesis**. They act as structural analogs of the D-Ala-D-Ala peptide terminus. They bind to and inhibit **Penicillin-Binding Proteins (PBPs)**, specifically the enzyme **transpeptidase**. This prevents the cross-linking of peptidoglycan chains, leading to a structurally weak cell wall. Consequently, the high internal osmotic pressure causes the bacteria to swell and burst (**Bactericidal action**). **Analysis of Incorrect Options:** * **B. Leakage from cell membrane:** This is the mechanism of **Polypeptide antibiotics** (e.g., Polymyxin B, Colistin) and **Antifungals** (e.g., Amphotericin B, Nystatin), which disrupt the integrity of the cytoplasmic membrane. * **C. Protein synthesis:** This is the mechanism for drugs like **Aminoglycosides, Tetracyclines, Macrolides, and Chloramphenicol**, which target the 30S or 50S ribosomal subunits. * **D. DNA gyrase:** This is the mechanism of **Fluoroquinolones** (e.g., Ciprofloxacin), which inhibit Topoisomerase II (DNA gyrase) and IV, preventing DNA replication. **NEET-PG High-Yield Pearls:** 1. **Autolysins:** Beta-lactams also activate bacterial autolysins, which further accelerate cell lysis. 2. **Time-Dependent Killing:** Beta-lactams exhibit time-dependent killing; their efficacy depends on the duration the drug concentration remains above the Minimum Inhibitory Concentration (MIC). 3. **Resistance:** The most common mechanism of resistance is the production of **Beta-lactamases**, which hydrolyze the beta-lactam ring. 4. **Synergy:** They are often combined with Aminoglycosides (e.g., in Enterococcal endocarditis) because cell wall inhibition facilitates the entry of aminoglycosides into the cell.

Question 40: Acquired multistep drug resistance can be seen in all except:

A. Erythromycin
B. Streptomycin (Correct Answer)
C. Tetracycline
D. Chloramphenicol

Explanation: ### Explanation The core concept tested here is the **pattern of development of bacterial resistance**. Resistance can be acquired via two primary kinetic patterns: **1. Why Streptomycin is the correct answer:** Streptomycin follows the **"Single-step" (Large-step) mutation** pattern, also known as the **Streptomycin-type resistance**. In this pattern, a single chromosomal mutation can result in a sudden, high degree of resistance, making the drug completely ineffective rapidly. This occurs because streptomycin binds to a specific site on the 30S ribosome; a single point mutation in the *rpsL* gene (encoding the S12 protein) can abolish this binding entirely. **2. Why the other options are incorrect:** * **Erythromycin, Tetracycline, and Chloramphenicol:** These drugs typically follow the **"Multi-step" (Small-step) mutation** pattern, also known as the **Penicillin-type resistance**. Resistance develops gradually through a series of successive mutations. Each mutation increases the Minimum Inhibitory Concentration (MIC) slightly, and clinical resistance is only achieved after multiple steps. **Clinical Pearls for NEET-PG:** * **Single-step resistance drugs:** Streptomycin, Rifampin, and Nalidixic acid. (Mnemonic: **SRN** - **S**udden **R**esistance **N**ow). * **Multi-step resistance drugs:** Penicillin, Erythromycin, Tetracyclines, and Chloramphenicol. * **Mechanism of Streptomycin Resistance:** Primarily due to chromosomal mutations altering the 30S ribosomal target or through the production of aminoglycoside-modifying enzymes (plasmid-mediated). * **Clinical implication:** Because resistance to Streptomycin and Rifampin develops so rapidly (single-step), they are almost never used as monotherapy for chronic infections like Tuberculosis.

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

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