Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 501: Which of the following drugs belongs to the antifolate group?

A. Erythromycin
B. Doxycycline
C. Pyrimethamine (Correct Answer)
D. Gentamicin

Explanation: ### Explanation **Correct Option: C. Pyrimethamine** **Mechanism of Action:** Antifolates are drugs that interfere with the synthesis or utilization of folic acid, which is essential for DNA and RNA synthesis in bacteria and parasites. **Pyrimethamine** is a potent inhibitor of the enzyme **Dihydrofolate Reductase (DHFR)**. By binding to DHFR, it prevents the reduction of dihydrofolate to tetrahydrofolate. It is primarily used in the treatment of toxoplasmosis and as an antimalarial agent (often in combination with Sulfadoxine). --- ### Analysis of Incorrect Options: * **A. Erythromycin:** This is a **Macrolide** antibiotic. It inhibits protein synthesis by binding to the **50S ribosomal subunit**, preventing translocation. * **B. Doxycycline:** This belongs to the **Tetracycline** class. It inhibits protein synthesis by binding to the **30S ribosomal subunit**, blocking the attachment of aminoacyl-tRNA to the A-site. * **D. Gentamicin:** This is an **Aminoglycoside**. It binds to the **30S ribosomal subunit**, causing mRNA misreading and inhibition of translocation. It is bactericidal, unlike most other protein synthesis inhibitors. --- ### NEET-PG High-Yield Pearls: 1. **Sequential Blockade:** The combination of a Sulfonamide (inhibits Dihydropteroate Synthase) and a DHFR inhibitor (like Pyrimethamine or Trimethoprim) results in a synergistic effect known as sequential blockade. 2. **DHFR Inhibitors to Remember:** * **Bacteria:** Trimethoprim * **Protozoa:** Pyrimethamine * **Humans (Cancer):** Methotrexate 3. **Side Effect:** Long-term use of Pyrimethamine can cause **megaloblastic anemia** due to folate deficiency. This is mitigated by co-administering **Folinic acid (Leucovorin rescue)**. 4. **Drug of Choice:** Pyrimethamine + Sulfadiazine is the treatment of choice for **Toxoplasmosis**.

Question 502: HIV strains resistant to which of the following drugs may show cross-resistance to Abacavir?

A. Lamivudine (Correct Answer)
B. Didanosine
C. Enfuvirtide
D. Raltegravir

Explanation: ### Explanation **Correct Option: A. Lamivudine** The development of resistance to Nucleoside Reverse Transcriptase Inhibitors (NRTIs) is primarily driven by specific mutations in the viral reverse transcriptase gene. The most significant mutation associated with **Lamivudine (3TC)** resistance is the **M184V** mutation. This specific mutation (substitution of methionine with valine at codon 184) not only causes high-level resistance to Lamivudine and Emtricitabine but also significantly reduces the viral susceptibility to **Abacavir**. Therefore, HIV strains that have developed resistance to Lamivudine via the M184V mutation exhibit clinical cross-resistance to Abacavir. **Analysis of Incorrect Options:** * **B. Didanosine:** While Didanosine is an NRTI, its primary resistance mutations (like L74V) do not typically confer the same level of predictable cross-resistance to Abacavir as the M184V mutation does. * **C. Enfuvirtide:** This is a **Fusion Inhibitor** that prevents the virus from entering the CD4 cell. Its mechanism and resistance patterns are entirely different from NRTIs. * **D. Raltegravir:** This is an **Integrase Strand Transfer Inhibitor (INSTI)**. It targets the viral integrase enzyme, meaning there is no cross-resistance with reverse transcriptase inhibitors like Abacavir. ### NEET-PG High-Yield Pearls: * **M184V Mutation:** High-yield for exams. It causes resistance to Lamivudine/Emtricitabine but interestingly **increases sensitivity** to Zidovudine (AZT) and Tenofovir. * **Abacavir Hypersensitivity:** Always screen for the **HLA-B*5701** allele before starting Abacavir to prevent life-threatening hypersensitivity reactions. * **K65R Mutation:** This mutation is associated with resistance to Tenofovir, Abacavir, and Lamivudine.

Question 503: Which of the following drugs can be used for the treatment of chloroquine-resistant malaria in children?

A. Chloroquine
B. Doxycycline
C. Tetracycline
D. Clindamycin (Correct Answer)

Explanation: The treatment of chloroquine-resistant *Plasmodium falciparum* malaria requires combination therapy. According to WHO and National Guidelines, when using Quinine for resistant malaria, it must be paired with a protein synthesis inhibitor to ensure complete parasite clearance and prevent relapse [1]. Why Clindamycin is the correct answer: Clindamycin is a lincosamide antibiotic that inhibits the 50S ribosomal subunit. In malaria treatment, it acts on the **apicoplast** (a plant-like organelle in the parasite) [1]. It is the **drug of choice for children (under 8 years)** and **pregnant women** when combined with Quinine for chloroquine-resistant malaria [1]. It is preferred in these groups because it lacks the bone and tooth-related toxicities associated with tetracyclines [1]. Why other options are incorrect: * **Chloroquine:** By definition, it is ineffective against chloroquine-resistant strains due to the *pfcrt* gene mutation which increases drug efflux from the parasite's food vacuole. * **Doxycycline & Tetracycline:** While these are highly effective against resistant malaria when combined with Quinine, they are **contraindicated in children under 8 years of age** [1]. They cause permanent tooth discoloration and inhibit bone growth by chelating calcium [1]. **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC) for Uncomplicated Malaria (India):** Artemisinin-based Combination Therapy (ACT). Specifically, Artesunate + Sulfadoxine-Pyrimethamine (AS+SP) is used nationally, except in North-Eastern states where Artemether-Lumefantrine is preferred. * **Radical Cure:** Primaquine is added to treat the liver stages (hypnozoites) in *P. vivax* and *P. ovale*, but it must be avoided in **G6PD deficiency** due to the risk of hemolysis. * **Mechanism of Clindamycin:** Targets the "delayed death" phenomenon in parasites by interfering with apicoplast replication [1].

Question 504: Which of the following is effective against dermatophytes?

A. Nystatin
B. Ketoconazole
C. Griseofulvin (Correct Answer)
D. Miconazole

Explanation: Explanation: Correct Option: C. GriseofulvinGriseofulvin is a fungistatic drug specifically used for infections caused by **dermatophytes** (*Trichophyton, Epidermophyton, and Microsporum*) [1]. Its mechanism of action involves binding to **fungal microtubules**, thereby inhibiting mitosis (metaphase arrest). * **The Key Concept:** Griseofulvin has a unique pharmacokinetic profile; it binds to newly formed **keratin** in the skin, hair, and nails, making them resistant to fungal invasion. This makes it highly effective for Tinea infections (Ringworm), especially **Tinea capitis**, where it remains a first-line systemic treatment [1]. Why other options are incorrect: * **A. Nystatin:** It is a polyene antibiotic (similar to Amphotericin B). It is **not effective against dermatophytes**. It is primarily used for *Candida* infections (moniliasis) of the skin, mouth, and vagina because it is not absorbed systemically [1]. * **B & D. Ketoconazole and Miconazole:** These are Imidazoles. While they have a broad spectrum that includes dermatophytes, they are primarily used **topically** for these infections [1]. Systemic Ketoconazole is rarely used now due to its side effect profile (hepatotoxicity and inhibition of steroid synthesis). In the context of "effectiveness" in classic pharmacology questions, Griseofulvin is the prototype drug specifically categorized by its narrow-spectrum activity against dermatophytes. High-Yield Clinical Pearls for NEET-PG: * **Absorption:** Griseofulvin absorption is significantly increased when taken with a **fatty meal**. * **Inducer:** It is a potent **Cytochrome P450 inducer**, which can decrease the efficacy of warfarin and oral contraceptives [2]. * **Disulfiram-like reaction:** Griseofulvin can cause a reaction when consumed with alcohol. * **Contraindication:** It is contraindicated in patients with **Porphyria** and systemic lupus erythematosus (SLE) [2].

Question 505: Which antitubercular drug can cause transient memory loss?

A. Ethionamide
B. Isoniazid (Correct Answer)
C. Ethambutol
D. Pyrazinamide

Explanation: **Explanation:** **Isoniazid (INH)** is the correct answer. The underlying mechanism for its neurotoxic side effects is its structural similarity to **Pyridoxine (Vitamin B6)**. Isoniazid increases the excretion of Pyridoxine and inhibits the enzyme *pyridoxine phosphokinase*, which is essential for converting B6 into its active form (Pyridoxal-5-phosphate). A deficiency in active Vitamin B6 leads to decreased synthesis of neurotransmitters like GABA. This manifests clinically as peripheral neuropathy (most common), but can also cause **Central Nervous System (CNS) effects** including psychosis, convulsions, and **transient memory loss** (amnesia). These effects are reversible with the administration of prophylactic Pyridoxine (10–50 mg/day). **Why other options are incorrect:** * **Ethionamide:** While it is chemically related to Isoniazid and can cause peripheral neuropathy or hepatotoxicity, it is not typically associated with transient memory loss. * **Ethambutol:** Its hallmark toxicity is **Optic Neuritis**, leading to decreased visual acuity and red-green color blindness. It does not cross the blood-brain barrier significantly. * **Pyrazinamide:** Its primary adverse effects are **Hepatotoxicity** and **Hyperuricemia** (leading to gouty arthritis) due to inhibition of uric acid excretion. **High-Yield Clinical Pearls for NEET-PG:** * **Isoniazid (INH):** Most common side effect is peripheral neuropathy; most serious is hepatitis. It is a potent **enzyme inhibitor**. * **Fast vs. Slow Acetylators:** INH is metabolized by acetylation. Slow acetylators are more prone to neuropathy, while fast acetylators may require higher doses. * **Sideroblastic Anemia:** INH can also cause this due to interference with heme synthesis (which requires B6).

Question 506: All of the following drugs have been used in the treatment of Kala-azar, except?

A. Sodium stibogluconate
B. Clindamycin (Correct Answer)
C. Amphotericin B
D. Pentamidine

Explanation: **Explanation:** Kala-azar (Visceral Leishmaniasis), caused by *Leishmania donovani*, requires specific antiprotozoal therapy. **Clindamycin (Option B)** is a lincosamide antibiotic primarily used for anaerobic bacterial infections and certain protozoa like *Plasmodium* (Malaria) or *Toxoplasma*. It has **no clinical efficacy** against *Leishmania* species and is therefore the correct "except" choice. **Analysis of other options:** * **Sodium Stibogluconate (Option A):** A pentavalent antimonial (SbV) that was traditionally the first-line treatment. However, its use is now limited in regions like Bihar, India, due to widespread resistance. * **Amphotericin B (Option C):** Currently the **drug of choice** for Kala-azar in India. Liposomal Amphotericin B (AmBisome) is preferred due to its high efficacy (single-dose therapy) and lower toxicity compared to the conventional deoxycholate form. * **Pentamidine (Option D):** An aromatic diamidine used as a second-line agent when antimonials fail. Its use has declined due to significant toxicities, including nephrotoxicity and insulin-dependent diabetes mellitus. **High-Yield Clinical Pearls for NEET-PG:** * **Miltefosine:** The first and only **oral** drug approved for Kala-azar. It is teratogenic (requires contraception for 3 months post-treatment). * **Paromomycin:** An aminoglycoside antibiotic that is also effective against Leishmaniasis when given intramuscularly. * **Drug of Choice (India):** Liposomal Amphotericin B (10 mg/kg single dose). * **Post-Kala-azar Dermal Leishmaniasis (PKDL):** Occurs in 5-10% of treated patients; usually requires longer courses of treatment.

Question 507: Which one of the following is primarily bacteriostatic?

A. Ciprofloxacin
B. Chloramphenicol (Correct Answer)
C. Vancomycin
D. Rifampicin

Explanation: The distinction between bacteriostatic and bactericidal agents is a high-yield concept in pharmacology. **Bacteriostatic** agents inhibit the growth and reproduction of bacteria without killing them immediately, relying on the host’s immune system to clear the infection. **Bactericidal** agents directly kill the bacteria. 1. Why Chloramphenicol is Correct: Chloramphenicol is a broad-spectrum antibiotic that acts by binding to the **50S ribosomal subunit**, inhibiting the enzyme peptidyl transferase. This prevents protein synthesis [2]. In most clinical scenarios, this inhibition is reversible, making the drug **primarily bacteriostatic** [1,2,3]. (Note: It can be bactericidal against specific highly sensitive organisms like *H. influenzae* and *N. meningitidis*) [1]. 2. Why the Other Options are Incorrect: * **Ciprofloxacin (Option A):** A Fluoroquinolone that inhibits DNA Gyrase and Topoisomerase IV. This leads to irreversible DNA damage and rapid cell death (Bactericidal). * **Vancomycin (Option B):** A Glycopeptide that inhibits cell wall synthesis by binding to the D-Ala-D-Ala terminus of nascent peptidoglycan. Cell wall inhibitors are typically Bactericidal. * **Rifampicin (Option D):** Inhibits DNA-dependent RNA polymerase, preventing transcription. It is a potent Bactericidal drug, especially important in Mycobacterial infections. Clinical Pearls for NEET-PG: * **Mnemonic for Bacteriostatic drugs:** "**MS. COLT**" (**M**acrolides, **S**ulfonamides, **C**hloramphenicol, **O**xazolidinones/Linezolid, **L**incosamides/Clindamycin, **T**etracyclines). * **Chloramphenicol Toxicity:** Watch for **Gray Baby Syndrome** (due to deficient glucuronidation in neonates) and **Aplastic Anemia** (idiosyncratic reaction). * **Rule of Thumb:** Most protein synthesis inhibitors are bacteriostatic (except Aminoglycosides), while cell wall inhibitors and DNA-targeting drugs are bactericidal [3].

Question 508: What is the mechanism of action of Triazoles?

A. Inhibits ergosterol biosynthesis (Correct Answer)
B. Inhibits tubulin
C. Inhibits glucan synthesis
D. Inhibits cell wall synthesis

Explanation: **Explanation:** **Mechanism of Action (Option A):** Triazoles (e.g., Fluconazole, Itraconazole, Voriconazole) are antifungal agents that act by **inhibiting the enzyme 14-α-demethylase**, a cytochrome P450-dependent enzyme. This enzyme is responsible for converting lanosterol to **ergosterol**, which is a vital component of the fungal cell membrane. The depletion of ergosterol disrupts membrane integrity and increases permeability, leading to fungal cell death (fungistatic/fungicidal effect). **Analysis of Incorrect Options:** * **Option B (Inhibits tubulin):** This is the mechanism of **Griseofulvin**, which interferes with microtubule function and spindle formation during fungal mitosis. * **Option C (Inhibits glucan synthesis):** This describes **Echinocandins** (e.g., Caspofungin). They inhibit β-(1,3)-D-glucan synthase, disrupting the fungal cell wall. * **Option D (Inhibits cell wall synthesis):** While glucan synthesis inhibitors (Echinocandins) target the cell wall, Triazoles specifically target the cell **membrane** components. **High-Yield Clinical Pearls for NEET-PG:** * **Fluconazole:** Drug of choice for Cryptococcal meningitis (maintenance) and Candidiasis; has the best CNS penetration. * **Voriconazole:** Drug of choice for **Invasive Aspergillosis**. A unique side effect is transient **visual disturbances** (photopsia). * **Itraconazole:** Drug of choice for Histoplasmosis, Blastomycosis, and Sporotrichosis. * **Resistance:** Fungal resistance to azoles often occurs via mutations in the *ERG11* gene or through efflux pumps. * **Drug Interactions:** Because they inhibit mammalian CYP450 enzymes (though less than Imidazoles), they can increase levels of drugs like Warfarin and Phenytoin.

Question 509: What is the drug of choice for symptomatic amoebiasis during pregnancy?

A. No treatment
B. Metronidazole
C. Diloxanide furoate (Correct Answer)
D. Diiodohydroxyquin

Explanation: **Explanation:** The management of amoebiasis in pregnancy requires a balance between therapeutic efficacy and fetal safety. Amoebiasis is caused by *Entamoeba histolytica*, which can present as asymptomatic colonization, intestinal disease, or extraintestinal (hepatic) abscesses. **Why Diloxanide Furoate is the Correct Choice:** Diloxanide furoate is a **luminal amoebicide**. In the context of symptomatic intestinal amoebiasis during pregnancy, it is considered the drug of choice, particularly in the second and third trimesters. It acts directly in the bowel lumen to eradicate cysts. While many clinicians use Metronidazole for severe symptoms, standard guidelines (and frequently tested NEET-PG patterns) prioritize luminal agents like Diloxanide furoate or Paromomycin due to their minimal systemic absorption, making them safer for the fetus. **Analysis of Incorrect Options:** * **A. No treatment:** Symptomatic amoebiasis must be treated to prevent complications like amoebic colitis, perforation, or liver abscess, which carry high maternal and fetal morbidity. * **B. Metronidazole:** Although highly effective against tissue trophozoites, Metronidazole is generally **avoided in the first trimester** due to theoretical concerns regarding teratogenicity (it is Category B, but crosses the placenta). It is reserved for severe invasive disease or hepatic abscess where the benefit outweighs the risk. * **D. Diiodohydroxyquin (Iodoquinol):** This is also a luminal amoebicide but is generally avoided in pregnancy due to the potential risk of optic neuritis and interference with thyroid function tests. **NEET-PG High-Yield Pearls:** * **Drug of Choice (General):** Metronidazole followed by a luminal agent (to prevent relapse). * **Luminal Amoebicides:** Diloxanide furoate, Paromomycin, Iodoquinol. * **Paromomycin:** Often cited in international literature as the safest luminal agent in pregnancy because it is not absorbed from the GIT at all. * **Asymptomatic Cyst Passers:** Treatment is mandatory to prevent the spread and potential invasion; Diloxanide furoate is the standard treatment.

Question 510: A resident doctor sustained a needlestick injury while sampling blood of a patient who is HIV positive. A decision is taken to offer him post-exposure prophylaxis. Which one of the following would be the best recommendation?

A. Zidovudine + Lamivudine for 4 weeks
B. Zidovudine + Lamivudine + Nevirapine for 4 weeks
C. Zidovudine + Lamivudine + Indinavir for 4 weeks (Correct Answer)
D. Zidovudine + Stavudine + Nevirapine for 4 weeks

Explanation: ### Explanation **1. Why Option C is Correct:** The management of occupational exposure to HIV (Needlestick Injury) follows the principle of **Post-Exposure Prophylaxis (PEP)**. According to the classic guidelines (on which this specific question is based), the "Expanded Regimen" is preferred for high-risk exposures (e.g., source patient is HIV positive). This regimen typically consists of **two Nucleoside Reverse Transcriptase Inhibitors (NRTIs)** and **one Protease Inhibitor (PI)**. * **Zidovudine (AZT) + Lamivudine (3TC)** form the backbone. * **Indinavir** (or Lopinavir/Ritonavir) is added as the third drug to increase potency. The duration of treatment is strictly **4 weeks (28 days)** to ensure the elimination of any potential viral replication. **2. Why Other Options are Incorrect:** * **Option A:** This is a "Basic Regimen." While used for low-risk exposures (e.g., solid needle, superficial injury), it is less effective than the expanded regimen when the source is known to be HIV positive. * **Option B & D:** These include **Nevirapine** (an NNRTI). Nevirapine is strictly **contraindicated** in PEP because it carries a high risk of severe hepatotoxicity and Stevens-Johnson Syndrome (SJS) in HIV-negative individuals receiving it for prophylaxis. * **Option D:** Combining Zidovudine and Stavudine is pharmacologically irrational as they compete for the same phosphorylation pathway (antagonism). **3. Clinical Pearls for NEET-PG:** * **Timing:** PEP should be started as soon as possible, ideally within **2 hours**, and definitely within **72 hours** of exposure. * **Current NACO/WHO Guidelines:** Modern PEP has shifted toward **Tenofovir (TDF) + Lamivudine (3TC) + Dolutegravir (DTG)** as the preferred first-line regimen due to better tolerability. * **Monitoring:** Baseline HIV testing of the healthcare worker is essential to rule out pre-existing infection. Repeat testing is done at 6 weeks, 12 weeks, and 6 months.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

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

Practice Questions

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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