Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 651: Non-nucleoside reverse transcriptase inhibitors (NNRTIs) include all of the following EXCEPT?

A. Nevirapine
B. Delavirdine
C. Etravirine
D. Lamivudine (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Lamivudine**. **1. Understanding the Concept** Antiretroviral drugs are classified based on their mechanism of action against HIV. Both **NRTIs** (Nucleoside Reverse Transcriptase Inhibitors) and **NNRTIs** (Non-Nucleoside Reverse Transcriptase Inhibitors) target the viral enzyme reverse transcriptase, but they do so differently: * **NRTIs (e.g., Lamivudine):** These are prodrugs that act as structural analogs of native nucleosides/nucleotides. They compete for the active site and cause **chain termination** once incorporated into the viral DNA. * **NNRTIs (e.g., Nevirapine):** These are non-competitive inhibitors that bind to a specific **allosteric site** (hydrophobic pocket) on the enzyme, causing a conformational change that inhibits its activity. They do *not* require phosphorylation or cause chain termination. **2. Analysis of Options** * **Nevirapine (A) & Delavirdine (B):** These are **1st Generation NNRTIs**. Nevirapine is notable for its use in preventing mother-to-child transmission (though largely replaced by newer regimens) and its risk of severe hepatotoxicity and Stevens-Johnson Syndrome (SJS). * **Etravirine (C):** This is a **2nd Generation NNRTI**, specifically designed to be effective against HIV strains that have developed resistance to 1st generation NNRTIs. * **Lamivudine (D):** This is an **NRTI** (Cytidine analog). It is a cornerstone of ART and is also used in the treatment of Hepatitis B. **3. NEET-PG Clinical Pearls** * **Mnemonic for NNRTIs:** "The **DEN**" (**D**elavirdine, **E**favirenz, **N**evirapine) or drugs with "**vir**" in the middle (Etra**vir**ine, Rilpi**vir**ine). * **Efavirenz:** The most common NNRTI; known for CNS side effects (vivid dreams, psychosis) and is generally avoided in the first trimester of pregnancy (teratogenic risk). * **Nevirapine:** Requires a "lead-in" dose to reduce the incidence of skin rash. * **Resistance:** A single mutation (K103N) can lead to high-level resistance across most 1st generation NNRTIs.

Question 652: Clindamycin acts by inhibiting which of the following processes?

A. Cell wall synthesis
B. Protein synthesis (Correct Answer)
C. Cell membrane synthesis
D. Glucose utilization

Explanation: **Explanation:** **Mechanism of Action:** Clindamycin is a lincosamide antibiotic that inhibits **protein synthesis** by binding to the **50S ribosomal subunit**. Specifically, it interferes with the formation of initiation complexes and the aminoacyl translocation reactions. Its mechanism is very similar to that of Macrolides (e.g., Erythromycin), leading to competitive inhibition if used together. **Analysis of Options:** * **Option A (Cell wall synthesis):** This is the mechanism for Beta-lactams (Penicillins, Cephalosporins), Vancomycin, and Bacitracin. Clindamycin does not affect the peptidoglycan layer. * **Option C (Cell membrane synthesis):** Drugs like Polymyxins (Daptomycin, Colistin) and Amphotericin B act by disrupting cell membrane integrity. * **Option D (Glucose utilization):** This is not a standard mechanism for primary antibacterial agents; drugs like Nitazoxanide (antiprotozoal) may interfere with energy metabolism, but it is irrelevant to Clindamycin. **High-Yield Clinical Pearls for NEET-PG:** 1. **Spectrum:** Excellent activity against **Gram-positive cocci** (including MRSA) and **Anaerobes** (e.g., *Bacteroides fragilis*). 2. **Clinical Use:** It is the drug of choice for anaerobic infections **above the diaphragm** (e.g., aspiration pneumonia, lung abscess). For infections below the diaphragm, Metronidazole is preferred. 3. **Adverse Effect:** Clindamycin is classically associated with **Pseudomembranous colitis** caused by *Clostridioides difficile* overgrowth. 4. **D-Test:** Used to detect inducible clindamycin resistance in Staphylococci that appear erythromycin-resistant but clindamycin-sensitive on initial testing.

Question 653: All of the following drugs are contraindicated in G6PD deficiency EXCEPT:

A. Ciprofloxacin (Correct Answer)
B. Primaquine
C. Dapsone
D. Sulfonamide

Explanation: **Explanation:** Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is an X-linked recessive disorder where red blood cells lack the enzyme necessary to maintain levels of reduced glutathione. Without glutathione, RBCs cannot neutralize oxidative stress, leading to hemoglobin denaturation (Heinz bodies) and acute hemolysis when exposed to certain oxidizing agents. **Why Ciprofloxacin is the Correct Answer:** While older fluoroquinolones (like Nalidixic acid) are classic triggers for hemolysis in G6PD deficiency, modern fluoroquinolones like **Ciprofloxacin** are generally considered safe. Although some older literature listed them as risks, current clinical evidence and hematology guidelines (such as those from the G6PD Deficiency Association) categorize Ciprofloxacin as a drug with "low to no risk" of hemolysis at standard therapeutic doses. **Analysis of Incorrect Options:** * **Primaquine:** This is the most notorious trigger. It is an absolute contraindication in G6PD deficiency as it causes severe oxidative damage. Patients must be screened for G6PD levels before starting radical cure for *P. vivax*. * **Dapsone:** A potent oxidant drug used in Leprosy. It causes dose-related hemolysis even in normal individuals, but in G6PD-deficient patients, it can cause life-threatening hemolytic anemia. * **Sulfonamides:** Drugs like Sulfamethoxazole (in Co-trimoxazole) are classic oxidative triggers that must be avoided. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for G6PD Triggers:** "**S**ell **D**apsone **P**rime **N**ow" (**S**ulfonamides, **D**apsone, **P**rimaquine, **N**itrofurantoin). * **Other Triggers:** Fava beans (Favism), Naphthalene balls, and infections (the most common cause of hemolysis). * **Diagnosis:** Peripheral smear shows **"Bite cells"** (degmacytes) and **"Heinz bodies"** (denatured hemoglobin) visualized with supravital stains like Crystal Violet.

Question 654: What is the drug of choice for hookworm infestation?

A. Piperazine citrate
B. Bephenium hydroxynaphthoate
C. Mebendazole
D. Albendazole (Correct Answer)

Explanation: **Explanation:** **Albendazole** is the drug of choice (DOC) for hookworm infestations (*Ancylostoma duodenale* and *Necator americanus*). Its mechanism of action involves inhibiting microtubule synthesis by binding to **β-tubulin**, which leads to glucose depletion and the eventual death of the parasite. **Why Albendazole is the Correct Choice:** Albendazole is preferred over other anthelmintics due to its high efficacy (single dose of 400 mg), broad-spectrum activity, and minimal systemic absorption, which reduces host toxicity. It is also the DOC for hydatid disease and neurocysticercosis. **Analysis of Incorrect Options:** * **Piperazine citrate:** It causes flaccid paralysis of worms by blocking ACh at the neuromuscular junction. While effective for *Ascaris*, it is **ineffective** against hookworms. * **Bephenium hydroxynaphthoate:** Once used for hookworms, it is now obsolete due to its bitter taste, frequent gastrointestinal side effects (nausea/vomiting), and the availability of superior agents like benzimidazoles. * **Mebendazole:** While highly effective against hookworms, it typically requires a **3-day course** (100 mg BID), making it less convenient than the single-dose regimen of Albendazole, which ensures better patient compliance. **High-Yield Clinical Pearls for NEET-PG:** * **Hookworm & Anemia:** Hookworm infestation is a leading cause of **Iron Deficiency Anemia** in the tropics due to chronic intestinal blood loss. * **Pregnancy:** Albendazole is generally avoided in the first trimester (embryotoxic in animals). * **Cutaneous Larva Migrans:** Albendazole is also the DOC for this condition, often caused by non-human hookworm species. * **Mass Drug Administration (MDA):** Under the National Deworming Day initiative, Albendazole is the drug used for mass treatment in schools.

Question 655: Docosanols is used as:

A. Antifungal
B. Antiviral (Correct Answer)
C. Analgesic
D. Antiemetic

Explanation: **Explanation:** **Docosanol** is a unique **antiviral agent** primarily used for the topical treatment of recurrent **Herpes Simplex Virus (HSV-1)** infections, commonly known as cold sores or fever blisters. **1. Why it is the Correct Answer:** Unlike most antivirals (like Acyclovir) which act as nucleoside analogs to inhibit viral DNA synthesis, Docosanol has a distinct **mechanism of action**. It is a saturated 22-carbon fatty alcohol that **inhibits the fusion** between the host cell plasma membrane and the HSV envelope. By preventing viral entry into the cell, it stops viral replication at the earliest stage. It does not have direct virucidal activity. **2. Why the Other Options are Incorrect:** * **Antifungal:** Antifungals (e.g., Fluconazole, Amphotericin B) target fungal cell membranes (ergosterol) or cell walls. Docosanol has no activity against fungal pathogens. * **Analgesic:** While Docosanol may reduce the pain associated with a healing cold sore by shortening the duration of the lesion, it possesses no intrinsic pain-relieving properties. * **Antiemetic:** Antiemetics (e.g., Ondansetron) act on neurotransmitter receptors (5-HT3, D2) to prevent vomiting; Docosanol has no systemic effect on the emetic center. **NEET-PG High-Yield Pearls:** * **FDA Status:** It is the only **Over-the-Counter (OTC)** topical antiviral cream approved by the FDA for herpes labialis. * **Clinical Timing:** For maximum efficacy, it must be applied during the **prodromal phase** (tingling/burning sensation) before the vesicle appears. * **Resistance:** Because it targets host cell entry rather than viral enzymes, resistance to Docosanol is less likely to develop compared to Acyclovir.

Question 656: What is the primary site of action for amphotericin B?

A. Ribosomes
B. Cell wall
C. Cell membrane (Correct Answer)
D. Protein synthesis

Explanation: **Explanation:** **Mechanism of Action (Why C is correct):** Amphotericin B is a polyene antifungal that targets the fungal **cell membrane**. It has a high affinity for **ergosterol**, the primary sterol in fungal membranes (analogous to cholesterol in humans). Amphotericin B molecules aggregate to form transmembrane **pores/channels**. These pores increase membrane permeability, leading to the leakage of intracellular ions (like Potassium) and small molecules, ultimately resulting in cell death (fungicidal action). **Analysis of Incorrect Options:** * **A & D (Ribosomes/Protein Synthesis):** These are the targets for many antibacterial agents (e.g., Aminoglycosides, Tetracyclines, Macrolides). Fungal protein synthesis is not the primary target of Amphotericin B. * **B (Cell Wall):** The fungal cell wall (composed of chitin and glucans) is the target for **Echinocandins** (e.g., Caspofungin), which inhibit beta-glucan synthesis. Amphotericin B acts deeper, on the lipid bilayer membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Spectrum:** It is the "Gold Standard" for most systemic fungal infections (Mucormycosis, Cryptococcal meningitis, Visceral Leishmaniasis). * **Resistance:** Occurs due to a decrease in the ergosterol content of the fungal membrane. * **Toxicity:** The most common dose-limiting toxicity is **Nephrotoxicity** (causes distal renal tubular acidosis and hypokalemia). * **Liposomal Amphotericin B:** Developed to reduce nephrotoxicity by targeting the drug specifically to fungal cells and sparing human renal cells. * **Infusion Reaction:** "Shake and bake" (fever, chills, rigors) is common during administration.

Question 657: Which of the following antimalarial agents is most commonly associated with acute hemolytic reaction in patients with glucose-6-phosphate dehydrogenase deficiency?

A. Chloroquine
B. Clindamycin
C. Mefloquine
D. Primaquine (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Primaquine** Primaquine is an 8-aminoquinoline that acts as a potent oxidizing agent. In individuals with **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**, the red blood cells (RBCs) cannot generate sufficient NADPH to maintain a pool of reduced glutathione. Reduced glutathione is essential for neutralizing oxidative stress. When exposed to Primaquine, the hemoglobin undergoes oxidative denaturation, forming **Heinz bodies**, which lead to membrane damage and subsequent **acute intravascular hemolysis**. This makes G6PD screening mandatory before initiating Primaquine therapy. **Incorrect Options:** * **A. Chloroquine:** While it is the drug of choice for sensitive malaria, it does not possess significant oxidizing properties and is generally safe in G6PD-deficient patients. * **B. Clindamycin:** An antibiotic used as an adjunct in malaria (especially in pregnancy), it does not cause oxidative stress or hemolysis. * **C. Mefloquine:** Primarily associated with neuropsychiatric side effects (vivid dreams, psychosis, seizures) and cardiac conduction issues, but not acute hemolysis in G6PD deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Primaquine Spectrum:** It is the only drug effective against **hypnozoites** (latent liver stages) of *P. vivax* and *P. ovale*, preventing relapse (Radical Cure). * **Gametocidal Activity:** It is highly effective against gametocytes of all species, including *P. falciparum*, thereby limiting transmission. * **Tafenoquine:** A newer long-acting analog of Primaquine that also requires G6PD screening. * **Contraindication:** Primaquine is strictly contraindicated in **pregnancy** because the G6PD status of the fetus cannot be determined, risking fetal hemolysis.

Question 658: Amphotericin B causes deficiency of which electrolyte?

A. Sodium
B. Calcium
C. Potassium (Correct Answer)
D. Chloride

Explanation: **Explanation:** **Mechanism of Hypokalemia:** Amphotericin B is a polyene antifungal that acts by binding to ergosterol in the fungal cell membrane, creating pores. However, it also has a partial affinity for **cholesterol** in human cell membranes, particularly in the renal tubules. When Amphotericin B binds to the distal tubular cells, it increases the permeability of the membrane. This leads to a "leak" of intracellular ions. Since the concentration of **Potassium ($K^+$)** is much higher inside the cell than in the tubular lumen, it leaks out down its concentration gradient, leading to significant **hypokalemia**. Additionally, it causes distal Renal Tubular Acidosis (Type 1 RTA), which further exacerbates potassium loss. **Analysis of Incorrect Options:** * **A. Sodium:** While Amphotericin B can cause some degree of natriuresis, the primary and most clinically significant electrolyte disturbance is the loss of potassium and magnesium. * **B. Calcium:** Amphotericin B does not typically cause significant fluctuations in serum calcium levels. * **D. Chloride:** Chloride levels are not directly targeted; however, a hyperchloremic metabolic acidosis may occur secondary to Type 1 RTA, but it is not the primary deficiency. **NEET-PG High-Yield Pearls:** 1. **Magnesium Loss:** Along with Potassium, **Hypomagnesemia** is a very common and characteristic side effect of Amphotericin B. 2. **Nephrotoxicity:** This is the dose-limiting toxicity. It can be minimized by "salt loading" (infusing normal saline before the drug). 3. **Liposomal Amphotericin B:** This formulation is preferred as it significantly reduces nephrotoxicity and electrolyte imbalances by targeting the drug more specifically to fungal cells. 4. **Infusion Reactions:** Often causes "shake and bake" symptoms (fever, chills, rigors). Pre-medication with NSAIDs or steroids is common.

Question 659: In the treatment of intestinal and extraintestinal amoebiasis, which of the following drugs is useful?

A. Diloxanide
B. Metronidazole (Correct Answer)
C. Chloroquine
D. Cefaclor

Explanation: **Explanation:** Amoebiasis, caused by *Entamoeba histolytica*, is clinically categorized into intestinal (dysentery, colitis) and extraintestinal (most commonly liver abscess) forms. To treat both, a drug must achieve therapeutic concentrations in both the gut lumen/wall and systemic tissues. **Why Metronidazole is Correct:** Metronidazole is a **systemic (mixed) amoebicide**. It is highly effective against the trophozoite forms of the parasite in the intestinal wall, liver, and other tissues [1]. It is the drug of choice for both amoebic dysentery and amoebic liver abscess [2]. However, because it is rapidly absorbed from the small intestine, it may not reach high enough concentrations in the colon lumen to eradicate cysts; therefore, it is usually followed by a luminal amoebicide [1]. **Analysis of Incorrect Options:** * **Diloxanide (furoate):** This is a **luminal amoebicide** [2]. It is poorly absorbed and acts only on trophozoites in the bowel lumen. It is used for asymptomatic cyst passers but is ineffective against extraintestinal (systemic) disease. * **Chloroquine:** This is a **tissue amoebicide** that concentrates specifically in the liver. While it is useful for amoebic liver abscess (extraintestinal), it has no efficacy against intestinal amoebiasis. * **Cefaclor:** This is a second-generation cephalosporin antibiotic used for bacterial infections (like RTI or UTI). It has no activity against protozoa like *E. histolytica*. **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** Metronidazole or Tinidazole for symptomatic amoebiasis [1]. * **Luminal Amoebicides:** Diloxanide furoate, Iodoquinol, and Paromomycin (used to prevent relapse) [2]. * **Mechanism of Metronidazole:** Produces reactive nitro-radicals that damage protozoal DNA [1]. * **Key Side Effect:** Disulfiram-like reaction with alcohol and a metallic taste in the mouth.

Question 660: Pseudomonas is resistant to which of the following antimicrobial agents?

A. Vancomycin (Correct Answer)
B. Aztreonam
C. Ciprofloxacin
D. Polymyxin B

Explanation: ### Explanation **1. Why Vancomycin is the Correct Answer:** *Pseudomonas aeruginosa* is a Gram-negative rod. **Vancomycin** is a glycopeptide antibiotic that acts by inhibiting cell wall synthesis (binding to the D-Ala-D-Ala terminus of nascent peptidoglycan). Due to its **large molecular size**, Vancomycin cannot penetrate the outer membrane of Gram-negative bacteria to reach its target. Therefore, *Pseudomonas* (and almost all Gram-negative organisms) are **intrinsically resistant** to Vancomycin. **2. Why the Other Options are Incorrect:** * **B. Aztreonam:** This is a Monobactam. It is unique because it has **no activity against Gram-positives or anaerobes** but is highly effective against aerobic Gram-negative bacteria, including *Pseudomonas*. * **C. Ciprofloxacin:** This is a second-generation Fluoroquinolone. It is one of the few **orally available** drugs with potent activity against *Pseudomonas*. * **D. Polymyxin B:** These are "detergent-like" antibiotics that disrupt the outer membrane. They are often used as **last-resort drugs** for multi-drug resistant (MDR) *Pseudomonas* infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Anti-Pseudomonal Penicillins:** Piperacillin and Ticarcillin. * **Anti-Pseudomonal Cephalosporins:** Ceftazidime (3rd gen) and Cefepime (4th gen). * **Carbapenems:** Imipenem and Meropenem are effective, but **Ertapenem** has NO activity against *Pseudomonas* (a common "catch" in exams). * **Aminoglycosides:** Amikacin and Tobramycin are frequently used in combination therapy. * **Intrinsic Resistance:** Remember that *Pseudomonas* is naturally resistant to many drugs due to its efflux pumps and low-permeability outer membrane.

Practice by Chapter

Beta-Lactam Antibiotics

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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

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Antimicrobial Resistance

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