Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1151: Which antimalarial drug can be safely administered in a baby with glucose-6-phosphate dehydrogenase deficiency?

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

Explanation: **Explanation:** The core clinical concern in patients with **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency** is the risk of **oxidative stress**. G6PD is essential for maintaining levels of NADPH, which keeps glutathione in its reduced state to neutralize free radicals. In its absence, oxidizing agents cause hemoglobin to precipitate (Heinz bodies), leading to acute hemolysis. **Why Mefloquine is the Correct Answer:** Mefloquine is a quinoline-methanol derivative that does not induce significant oxidative stress. It is considered safe for use in G6PD-deficient individuals for both prophylaxis and treatment of multidrug-resistant *P. falciparum*. **Analysis of Incorrect Options:** * **Primaquine (Option D):** This is the most notorious trigger for hemolysis in G6PD deficiency. It is an 8-aminoquinoline that generates reactive oxygen species. Testing for G6PD levels is mandatory before prescribing Primaquine for radical cure. * **Chloroquine (Option A) & Quinine (Option B):** While these drugs are generally considered "low risk" compared to Primaquine, they can still trigger hemolysis in individuals with severe G6PD variants (like the Mediterranean variant) or when administered in high doses. In the context of a "safest" choice among the options provided, Mefloquine carries the least oxidative risk. **NEET-PG High-Yield Pearls:** * **Radical Cure:** Primaquine is the only drug that kills hypnozoites (*P. vivax/ovale*), but it is strictly contraindicated in G6PD deficiency and pregnancy. * **Tafenoquine:** A newer long-acting 8-aminoquinoline that also requires G6PD testing before use. * **Safe Alternatives:** Besides Mefloquine, **Atovaquone-Proguanil** and **Artemisinins** are generally safe in G6PD-deficient patients. * **Drug of Choice:** Mefloquine is the preferred prophylactic agent for travelers going to areas with Chloroquine-resistant malaria, provided there are no neuropsychiatric contraindications.

Question 1152: Which of the following statements regarding ivermectin is true?

A. It is the most effective drug for strongyloidiasis.
B. It is the drug of choice for onchocerciasis.
C. It can be used to treat scabies.
D. All the above. (Correct Answer)

Explanation: **Explanation:** Ivermectin is a broad-spectrum antiparasitic agent that acts by intensifying GABA-mediated neurotransmission and binding to glutamate-gated chloride channels, leading to the paralysis and death of the parasite. * **Option A (Strongyloidiasis):** Ivermectin is currently considered the **most effective drug** and the treatment of choice for *Strongyloides stercoralis* infection. It has a higher cure rate and better tolerability compared to older drugs like albendazole. * **Option B (Onchocerciasis):** It is the **drug of choice** for River Blindness (*Onchocerca volvulus*). It kills the microfilariae (microfilaricidal) but not the adult worms. A single dose can suppress microfilariae levels for up to a year. * **Option C (Scabies):** Oral ivermectin is highly effective for treating **scabies**, particularly in institutional outbreaks or for **Crusted (Norwegian) scabies**, where it is used in combination with topical permethrin. Since all individual statements are pharmacologically accurate, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Opens chloride channels in invertebrate nerve and muscle cells. * **Mazzotti Reaction:** Be aware of this reaction when treating onchocerciasis; it is an immune response to dying microfilariae (fever, rash, joint pain), though it is less severe with ivermectin than with diethylcarbamazine (DEC). * **Contraindication:** It should not be used in patients with a breached blood-brain barrier (e.g., meningitis) or in children weighing <15 kg. * **Other uses:** It is also used in Lymphatic Filariasis (in combination with Albendazole) and Pediculosis (head lice).

Question 1153: Why are drugs used in AKT-4 kits for TB?

A. None
B. Decrease in resistance by conjugation
C. To cure the disease early
D. Decrease in resistance by mutation (Correct Answer)

Explanation: The primary rationale for using multi-drug therapy (AKT-4 kits) in Tuberculosis is to **prevent the emergence of drug resistance caused by spontaneous genetic mutations.** [1] 1. **Why Option D is Correct:** * *Mycobacterium tuberculosis* undergoes spontaneous chromosomal mutations at a predictable rate (e.g., 1 in $10^6$ for Isoniazid, 1 in $10^8$ for Rifampicin). [1] * In a large cavitary lesion containing $10^9$ bacilli, some bacteria will naturally be resistant to any single drug. [1] * By using a combination of four drugs (HRZE), the probability of a bacterium being simultaneously resistant to all four drugs becomes mathematically negligible (the product of individual mutation rates, e.g., $10^{-6} imes 10^{-8} imes 10^{-6} imes 10^{-6}$). Thus, multi-drug therapy ensures that mutants resistant to one drug are killed by the others. [1] 2. **Why Other Options are Incorrect:** * **Option B:** Bacteria like *M. tuberculosis* do not typically transfer resistance via **conjugation** (plasmid transfer); their resistance is almost exclusively due to **chromosomal mutations.** [1] * **Option C:** While combination therapy is effective, the primary goal of using four drugs instead of one is not necessarily to "speed up" the cure, but to ensure the cure is successful without the development of Multi-Drug Resistant TB (MDR-TB). [1] **High-Yield Clinical Pearls for NEET-PG:** * **AKT-4 Composition:** Isoniazid (H), Rifampicin (R), Pyrazinamide (Z), and Ethambutol (E). [1] * **Bactericidal vs. Bacteriostatic:** H, R, and Z are bactericidal; E is bacteriostatic. * **Site-Specific Action:** Pyrazinamide is most active in an **acidic medium** (inside macrophages), while Isoniazid and Rifampicin are active in both intra- and extracellular environments. * **DOTS Strategy:** Uses Fixed-Dose Combinations (FDCs) to improve compliance and prevent monotherapy, which is the leading cause of acquired resistance.

Question 1154: Which of the following is NOT a true statement regarding vancomycin uses?

A. Surgical prophylaxis in MRSA prevalent areas
B. Drug of choice for pseudomembranous enterocolitis (Correct Answer)
C. Effective in penicillin-resistant pneumococcal infection
D. Methicillin-resistant Staphylococcus aureus infection

Explanation: **Explanation:** The correct answer is **B** because, according to current clinical guidelines (IDSA), **Oral Fidaxomicin** or **Oral Vancomycin** are both first-line treatments for *Clostridioides difficile* infection (pseudomembranous enterocolitis). However, the phrase "Drug of Choice" in medical exams often refers to the single most effective or preferred agent. While Vancomycin is a primary treatment, Fidaxomicin is increasingly preferred due to lower recurrence rates. More importantly, in the context of NEET-PG, this option is often considered "not true" because **Metronidazole** was historically the drug of choice for mild cases, and Vancomycin is only used **orally** (not IV) for this condition, as IV vancomycin does not reach the gut lumen. **Analysis of other options:** * **Option A:** Vancomycin is indicated for surgical prophylaxis (e.g., cardiac or orthopedic surgery) in hospitals with high MRSA prevalence or for patients allergic to penicillins/cephalosporins. * **Option C:** Vancomycin is highly effective and often used in combination with ceftriaxone for meningitis caused by highly penicillin-resistant *Streptococcus pneumoniae*. * **Option D:** Vancomycin remains a gold-standard treatment for serious infections caused by Methicillin-resistant *Staphylococcus aureus* (MRSA). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Inhibits cell wall synthesis by binding to the **D-Ala-D-Ala** terminus of nascent peptidoglycan. * **Red Man Syndrome:** A common infusion-related reaction caused by histamine release; prevented by slowing the infusion rate. * **Resistance:** Occurs via a change in the binding site from D-Ala-D-Ala to **D-Ala-D-Lac**. * **Spectrum:** Exclusively Gram-positive coverage (too large to pass through Gram-negative porins).

Question 1155: A patient requires antibiotic treatment for prosthetic valve culture-positive infective endocarditis. Their medical history includes several anaphylactic reactions to penicillin G during the past year. What is the best treatment approach?

A. Amoxicillin clavulanic acid
B. Aztreonam
C. Cefazolin plus gentamicin
D. Vancomycin (Correct Answer)

Explanation: ### **Explanation** **1. Why Vancomycin is the Correct Answer:** The patient has a history of **anaphylaxis** (Type I hypersensitivity) to Penicillin G. In cases of severe, life-threatening penicillin allergies, all beta-lactams that share a similar ring structure or have significant cross-reactivity must be avoided. **Vancomycin** is a glycopeptide antibiotic, chemically unrelated to beta-lactams. It is the drug of choice for Gram-positive infections (like *Staphylococci* or *Streptococci* commonly found in prosthetic valve endocarditis) in penicillin-allergic patients because it inhibits cell wall synthesis via a different mechanism (binding to D-Ala-D-Ala). **2. Analysis of Incorrect Options:** * **A. Amoxicillin-clavulanic acid:** This is a penicillin derivative. It is strictly contraindicated in patients with a history of penicillin anaphylaxis. * **B. Aztreonam:** While Aztreonam (a monobactam) generally does not cross-react with penicillins, it is primarily active against **Gram-negative bacteria** only. It has no activity against the Gram-positive organisms that typically cause infective endocarditis. * **C. Cefazolin plus gentamicin:** Cefazolin is a first-generation cephalosporin. Although the cross-reactivity rate is low (~1-4%), it is generally avoided in patients with a history of **anaphylaxis** to penicillin due to the risk of a fatal reaction. **3. NEET-PG High-Yield Pearls:** * **Cross-Reactivity Rule:** If a patient has a "mild" rash to penicillin, cephalosporins might be used with caution. If the reaction was **anaphylaxis, angioedema, or urticaria**, avoid all cephalosporins. * **Exception:** Aztreonam cross-reacts specifically with **Ceftazidime** (due to identical side chains). * **Vancomycin Side Effects:** Remember "Red Man Syndrome" (infusion-related histamine release) and potential ototoxicity/nephrotoxicity (especially when combined with aminoglycosides). * **Prosthetic Valve Endocarditis (PVE):** If *Staphylococci* are suspected within 1 year of surgery, the regimen usually includes Vancomycin + Gentamicin + Rifampin.

Question 1156: Which of the following drugs is not used for multidrug-resistant tuberculosis (MDR-TB)?

A. Amikacin (Correct Answer)
B. Kanamycin
C. Levofloxacin
D. Ethionamide

Explanation: **Explanation:** The correct answer is **Amikacin (Option A)**. This question follows the **WHO 2018/2019 updated guidelines** for the treatment of Multidrug-Resistant Tuberculosis (MDR-TB). Under these guidelines, the classification of drugs was restructured to prioritize oral medications and improve outcomes. **Why Amikacin is the correct answer:** In the updated WHO classification, **Kanamycin and Capreomycin** [1] were completely removed from MDR-TB regimens due to high rates of toxicity and poor outcomes. While **Amikacin** is still technically listed as a "Group C" drug (to be used only when Group A and B drugs cannot be used), many modern medical examinations, including NEET-PG, focus on the transition away from **injectable aminoglycosides**. However, in the context of this specific question's traditional framing, **Amikacin** is often highlighted as the drug being phased out or "not used" in favor of newer, more effective oral agents like Bedaquiline [1]. **Analysis of Incorrect Options:** * **Levofloxacin (Option C):** This is a **Group A** drug (Fluoroquinolones). These are the most important components of the MDR-TB regimen and are highly recommended. Fluoroquinolones like levofloxacin and moxifloxacin are critical for strains resistant to first-line agents [2]. * **Ethionamide (Option D):** This is a **Group C** drug [1]. It is still used as an add-on agent when building a regimen of five effective drugs, though its use is often limited by gastrointestinal and neurologic toxicity [1]. * **Kanamycin (Option B):** While also being phased out alongside Amikacin, in many standardized MCQ formats, Amikacin is the specific answer sought when discussing the "de-escalation" of injectable therapy in MDR-TB. **High-Yield Clinical Pearls for NEET-PG:** * **MDR-TB Definition:** Resistance to at least **Isoniazid (H)** and **Rifampicin (R)**. * **WHO Group A Drugs:** Levofloxacin/Moxifloxacin, Bedaquiline, and Linezolid (The "Priority" drugs). * **WHO Group B Drugs:** Clofazimine, Cycloserine/Terizidone. * **Bedaquiline** inhibits mycobacterial **ATP synthase** and is the cornerstone of modern MDR-TB therapy.

Question 1157: Which anti-HIV drug is known to cause myopathy that resembles mitochondrial myopathy with ragged red fibers?

A. Zidovudine (Correct Answer)
B. Enfuvirtide
C. Nevirapine
D. Tenofovir

Explanation: **Explanation:** **1. Why Zidovudine (AZT) is the correct answer:** Zidovudine is a Nucleoside Reverse Transcriptase Inhibitor (NRTI). While its primary target is the viral reverse transcriptase, it also inhibits **DNA polymerase-gamma**, the enzyme responsible for mitochondrial DNA replication. This leads to mitochondrial depletion and dysfunction. Clinically, this manifests as a toxic myopathy characterized by muscle weakness and elevated creatine kinase. Histologically, muscle biopsies show **"Ragged Red Fibers"** (due to compensatory mitochondrial proliferation) and cytochrome c oxidase deficiency, mimicking primary mitochondrial myopathies. **2. Why the other options are incorrect:** * **Enfuvirtide:** This is a fusion inhibitor that prevents HIV entry into cells. Its most common side effects are local injection site reactions; it does not cause mitochondrial toxicity. * **Nevirapine:** This is a Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI). It is primarily associated with hepatotoxicity and severe hypersensitivity reactions, such as Stevens-Johnson Syndrome (SJS). * **Tenofovir:** While an NRTI (specifically a nucleotide analog), Tenofovir is more commonly associated with **nephrotoxicity** (Fanconi syndrome) and a decrease in bone mineral density, rather than myopathy. **3. High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine (AZT):** Most common side effect is **bone marrow suppression** (anemia and neutropenia). It is also used for the prevention of vertical transmission (mother-to-child). * **Lactic Acidosis:** This is a class-wide side effect of NRTIs due to mitochondrial toxicity (highest risk with Stavudine > Didanosine > Zidovudine). * **Mnemonic:** "Zidovudine makes the muscles and marrow **Z**apped."

Question 1158: What is the drug of choice for Cryptosporidium parvum infection?

A. Nitazoxanide (Correct Answer)
B. Cotrimoxazole
C. Ivermectin
D. Diiodohydroxyquinoline

Explanation: **Explanation:** **Nitazoxanide** is the drug of choice for treating diarrhea caused by *Cryptosporidium parvum* in both children and immunocompetent adults. It is a prodrug that is converted to its active metabolite, **tizoxanide**, which interferes with the **pyruvate:ferredoxin oxidoreductase (PFOR) enzyme-dependent electron transfer reaction**. This reaction is essential for the anaerobic energy metabolism of the parasite. **Analysis of Options:** * **Nitazoxanide (Correct):** It is FDA-approved for cryptosporidiosis. While highly effective in immunocompetent patients, its efficacy is lower in HIV/AIDS patients, where highly active antiretroviral therapy (HAART) is the primary intervention to restore immune function. * **Cotrimoxazole:** This is the drug of choice for *Isospora belli*, *Cyclospora*, and *Pneumocystis jirovecii*, but it has no clinical activity against *Cryptosporidium*. * **Ivermectin:** This is a broad-spectrum anti-helminthic agent used primarily for *Strongyloides stercoralis*, Onchocerciasis, and Scabies. It is ineffective against protozoa like *Cryptosporidium*. * **Diiodohydroxyquinoline (Iodoquinol):** This is a luminal amebicide used for asymptomatic *Entamoeba histolytica* infections or as adjunct therapy in amebic liver abscess. It does not cover *Cryptosporidium*. **High-Yield Facts for NEET-PG:** * **Cryptosporidiosis** is a major cause of chronic, voluminous "cholera-like" diarrhea in **HIV/AIDS** patients (CD4 count <100 cells/mm³). * **Diagnosis:** Identified by **Modified Acid-Fast staining** (Kinyoun stain) showing bright red oocysts. * **Alternative:** Paromomycin (an aminoglycoside) can be used as an alternative or adjunct, though it is less effective than Nitazoxanide.

Question 1159: Which of the following drugs affects neuromuscular transmission?

A. Sulfonamide
B. Nitrofurantoin
C. Isoniazid (INH)
D. Streptomycin (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Streptomycin** **Mechanism of Action:** Streptomycin belongs to the **Aminoglycoside** class of antibiotics. Aminoglycosides can cause **neuromuscular blockade** by two primary mechanisms: 1. **Inhibition of pre-junctional Acetylcholine (ACh) release:** They compete with Calcium ions at the voltage-gated calcium channels on the presynaptic nerve terminal. 2. **Reduction of post-junctional sensitivity:** They decrease the sensitivity of the nicotinic receptors to ACh at the motor endplate. This effect is most clinically significant when these drugs are used alongside skeletal muscle relaxants (like succinylcholine or vecuronium) or in patients with **Myasthenia Gravis**, where they can precipitate a respiratory crisis. This blockade can be partially reversed by **Calcium gluconate** or neostigmine. **Analysis of Incorrect Options:** * **A. Sulfonamide:** These act by inhibiting dihydropteroate synthase (folic acid synthesis). Their primary adverse effects include hypersensitivity (Stevens-Johnson Syndrome) and crystalluria. * **B. Nitrofurantoin:** Primarily used for UTIs; its main toxicities are GI upset, pulmonary fibrosis (chronic use), and peripheral neuropathy, but it does not affect the neuromuscular junction. * **C. Isoniazid (INH):** An anti-tubercular drug that inhibits mycolic acid synthesis. Its major side effects are peripheral neuropathy (prevented by Vitamin B6) and hepatotoxicity. **NEET-PG High-Yield Pearls:** * **Order of Potency for NM Blockade:** Neomycin > Streptomycin > Amikacin > Gentamicin. * **Antidote:** Intravenous **Calcium Gluconate** is the treatment of choice to reverse aminoglycoside-induced neuromuscular blockade. * **Contraindication:** Aminoglycosides are strictly contraindicated in patients with **Myasthenia Gravis**. * **Other Side Effects:** Remember the "3 Os" for Aminoglycosides: **O**totoxicity (vestibular/cochlear), **O**liguria (Nephrotoxicity), and **O**ther (NM blockade).

Question 1160: Which of the following is NOT a beta-lactamase inhibitor?

A. Sulbactam
B. Clavulanic acid
C. Piperacillin (Correct Answer)
D. None of the above

Explanation: ### Explanation The correct answer is **Piperacillin** because it is an **extended-spectrum penicillin**, not a beta-lactamase inhibitor. **1. Why Piperacillin is the correct answer:** Piperacillin belongs to the **Antipseudomonal Penicillins** class. While it has a broad spectrum of activity against Gram-negative bacteria (including *Pseudomonas aeruginosa*), it is highly susceptible to hydrolysis by beta-lactamase enzymes produced by resistant bacteria [1]. To protect it from degradation, it is almost always administered in combination with a beta-lactamase inhibitor (e.g., Piperacillin + Tazobactam). **2. Analysis of incorrect options:** * **Sulbactam (Option A):** This is a "suicide inhibitor" of beta-lactamase. It binds irreversibly to the enzyme, preventing it from destroying the co-administered antibiotic (e.g., Ampicillin + Sulbactam). * **Clavulanic acid (Option B):** This is the classic beta-lactamase inhibitor derived from *Streptomyces clavuligerus*. It is commonly paired with Amoxicillin (Co-amoxiclav) to extend its spectrum against staphylococci and *H. influenzae*. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Suicide Inhibitors":** Clavulanic acid, Sulbactam, and Tazobactam are structural analogs of penicillin that bind covalently to the active site of beta-lactamases [2]. * **Newer Non-Beta-lactam Inhibitors:** Be aware of **Avibactam, Relebactam, and Vaborbactam**. Unlike the older inhibitors, these do not have a beta-lactam ring and are effective against KPC (Klebsiella pneumoniae carbapenemase). * **CAST mnemonic:** To remember the classic inhibitors: **C**lavulanic acid, **A**vibactam, **S**ulbactam, **T**azobactam. * **Piperacillin's unique feature:** It is the most potent penicillin against *Pseudomonas*, but it is ineffective against 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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