Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1051: Maraviroc acts as which of the following?

A. Reverse transcriptase inhibitor
B. Protease inhibitor
C. Entry inhibitor (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Maraviroc** is a specialized antiretroviral drug classified as an **Entry Inhibitor**. Its specific mechanism involves binding to the **CCR5 receptor** on the surface of human CD4+ T-cells. By acting as a **CCR5 coreceptor antagonist**, it prevents the HIV-1 gp120 protein from interacting with the host cell, thereby blocking the virus from entering the cell. **Analysis of Options:** * **Option A (Reverse Transcriptase Inhibitors):** These drugs (e.g., Zidovudine, Efavirenz) act intracellularly to prevent the conversion of viral RNA into DNA. Maraviroc acts extracellularly before the virus enters. * **Option B (Protease Inhibitors):** These drugs (e.g., Ritonavir, Atazanavir) act at the final stage of the viral life cycle by preventing the cleavage of precursor polypeptides, thus stopping the maturation of new virions. * **Option C (Correct):** Maraviroc prevents the initial "docking" and entry phase of the HIV life cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Tropism Requirement:** Maraviroc is only effective against **R5-tropic HIV** (viruses that use the CCR5 receptor). It is ineffective against X4-tropic (CXCR4) or dual-tropic viruses. A **Trofile assay** must be performed before starting treatment. * **Metabolism:** It is a substrate of **CYP3A4**; therefore, dosage adjustments are required when co-administered with CYP3A4 inhibitors (like Ritonavir) or inducers (like Rifampin). * **Difference from Enfuvirtide:** While both are entry inhibitors, **Enfuvirtide** is a fusion inhibitor that binds to the viral **gp41** subunit, whereas Maraviroc binds to the host **CCR5** receptor.

Question 1052: What is the difference in the action of Diethylcarbamazine (DEC) and Ivermectin in scrotal filariasis?

A. DEC is more effective on microfilariae than Ivermectin.
B. DEC acts only on microfilariae, while Ivermectin acts only on adult worms.
C. DEC acts on both microfilariae and adult worms, while Ivermectin acts on adult worms only.
D. DEC acts on adult worms, while Ivermectin acts on microfilariae. (Correct Answer)

Explanation: ### Explanation The management of lymphatic filariasis (caused by *Wuchereria bancrofti* or *Brugia malayi*) involves targeting two different stages of the parasite: the **microfilariae** (circulating larvae) and the **macrofilariae** (adult worms). **1. Why Option D is Correct:** * **Diethylcarbamazine (DEC):** It is the drug of choice for lymphatic filariasis because it possesses both microfilaricidal and **macrofilaricidal** (adulticidal) activity. In scrotal filariasis, where adult worms reside in the lymphatics, DEC is essential to kill the adult worms and prevent disease progression. * **Ivermectin:** It is a potent **microfilaricidal** agent. It works by paralyzing the microfilariae (via glutamate-gated chloride channels), facilitating their clearance by the host's immune system. However, it has **no significant effect on the adult worms.** **2. Why Other Options are Incorrect:** * **Option A:** Ivermectin is actually more potent and faster-acting against microfilariae than DEC, but it lacks the adulticidal effect. * **Option B:** This is the reverse of the actual mechanism. DEC is effective against both stages, whereas Ivermectin is limited to microfilariae. * **Option C:** While it correctly identifies DEC’s dual action, it incorrectly states that Ivermectin acts on adult worms. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mazzotti Reaction:** A severe inflammatory response (fever, rash, tachycardia) seen after DEC treatment, caused by the rapid death of microfilariae. It is most common in Onchocerciasis. * **Contraindication:** DEC is **contraindicated in Onchocerciasis** (River Blindness) because it can cause severe ocular inflammation and blindness. Ivermectin is the drug of choice for Onchocerciasis. * **Combination Therapy:** For Mass Drug Administration (MDA), the WHO recommends a triple therapy (IDA): **I**vermectin + **D**EC + **A**lbendazole. * **Albendazole:** Does not kill worms directly but inhibits embryogenesis in adult females.

Question 1053: Which of the following drugs is not used for the treatment of typhoid?

A. Chloramphenicol
B. Ciprofloxacin
C. Ceftriaxone
D. Cefixime (Correct Answer)

Explanation: **Explanation:** The treatment of Typhoid (Enteric) fever has evolved significantly due to the emergence of Multi-Drug Resistant (MDR) and Extensively Drug-Resistant (XDR) strains of *Salmonella typhi*. **Why Cefixime is the correct answer:** While Cefixime is an oral third-generation cephalosporin, it is generally **not recommended** as a primary treatment for typhoid fever in current clinical practice. Clinical trials have shown that Cefixime has higher failure rates and longer time-to-defervescence (time taken for fever to subside) compared to injectable Ceftriaxone or Azithromycin. In the context of competitive exams, it is often listed as the "incorrect" choice when compared to established first-line or historical agents. **Analysis of other options:** * **Chloramphenicol (A):** Historically the "Gold Standard" for typhoid. Although its use has declined due to bone marrow toxicity (Aplastic anemia) and the rise of MDR strains, it remains effective against sensitive strains. * **Ciprofloxacin (B):** A Fluoroquinolone that was the drug of choice for decades. However, due to increasing resistance (NALF - Nalidixic Acid Resistant *S. typhi*), its utility is now limited, though it is still used for sensitive cases. * **Ceftriaxone (C):** Currently the **Drug of Choice (DOC)** for complicated or hospitalized typhoid fever, especially in regions with high MDR/XDR prevalence. It is administered intravenously. **High-Yield Clinical Pearls for NEET-PG:** * **DOC for Uncomplicated Typhoid:** Oral Azithromycin. * **DOC for Complicated/Severe Typhoid:** IV Ceftriaxone. * **Carrier State:** Chronic carriers (bacteria in gallbladder) are treated with **Amoxicillin** or **Ciprofloxacin** for 4–6 weeks. If cholelithiasis is present, cholecystectomy may be required. * **XDR Typhoid:** Defined as resistance to chloramphenicol, ampicillin, trimethoprim-sulfamethoxazole, fluoroquinolones, and third-generation cephalosporins. **Carbapenems (Meropenem)** are used for XDR strains.

Question 1054: Voriconazole is not effective against which of the following?

A. Aspergillosis
B. Mucormycosis (Correct Answer)
C. Candida albicans
D. Candida tropicalis

Explanation: **Explanation:** The correct answer is **Mucormycosis**. Voriconazole is a second-generation triazole and a derivative of fluconazole. While it has an expanded spectrum compared to its predecessor, it has a significant clinical "gap": it lacks activity against the **Mucorales** order (the causative agents of Mucormycosis). **1. Why Mucormycosis is the correct answer:** Voriconazole is ineffective against Zygomycetes (Mucor and Rhizopus species). In fact, prolonged use of voriconazole in immunocompromised patients is a known risk factor for the development of breakthrough Mucormycosis. The drugs of choice for Mucormycosis are **Amphotericin B** (Liposomal) or newer azoles like **Isavuconazole** and **Posaconazole**. **2. Why other options are incorrect:** * **Aspergillosis:** Voriconazole is the **drug of choice (DOC)** for invasive Aspergillosis, offering better survival rates and fewer side effects than Amphotericin B. * **Candida species (C. albicans & C. tropicalis):** Voriconazole has excellent activity against most Candida species, including those resistant to fluconazole (like *C. krusei* and some strains of *C. glabrata*). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits 14-alpha-demethylase, preventing the synthesis of ergosterol. * **Side Effects (High Yield):** 1. **Visual disturbances:** Photopsia (flashing lights) or blurred vision (transient). 2. **Skin:** Photosensitivity and increased risk of squamous cell carcinoma. 3. **Neurological:** Hallucinations. * **Metabolism:** Exhibits non-linear kinetics (saturation kinetics). * **Spectrum Tip:** Remember that among azoles, only **Posaconazole** and **Isavuconazole** cover Mucorales.

Question 1055: A 10-year-old boy presents with acute bloody diarrhea and fever. What is the drug of choice for suspected Shigella infection?

A. Penicillin
B. Doxycycline
C. Azithromycin
D. Ciprofloxacin (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Ciprofloxacin** **Mechanism and Rationale:** Shigellosis (bacillary dysentery) is characterized by high fever, abdominal cramps, and bloody mucoid stools. According to current WHO guidelines and standard pharmacological practice, **Fluoroquinolones (specifically Ciprofloxacin)** are the first-line treatment for Shigella infection in both adults and children. Ciprofloxacin is highly effective because it achieves high concentrations in the intestinal mucosa and inhibits **DNA Gyrase (Topoisomerase II)** and **Topoisomerase IV**, leading to rapid bactericidal action against Gram-negative enterics. While there are concerns regarding cartilage toxicity in children, the benefits of treating invasive diarrhea outweigh the risks, and short-term use is considered safe. **Analysis of Incorrect Options:** * **A. Penicillin:** Shigella species are naturally resistant to Penicillin G. While Ampicillin was once used, widespread resistance has rendered it obsolete for empiric therapy. * **B. Doxycycline:** Tetracyclines are generally not used for Shigella due to high levels of plasmid-mediated resistance and potential side effects in children (tooth discoloration). * **C. Azithromycin:** This is considered a **second-line** agent or an alternative in areas with high fluoroquinolone resistance. While effective, Ciprofloxacin remains the primary recommendation in most standardized exams. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Ciprofloxacin (1st line); Azithromycin or Ceftriaxone (2nd line/Alternative). * **Avoid Antimotility Agents:** Drugs like Loperamide are **contraindicated** in Shigellosis as they can worsen the infection and increase the risk of Toxic Megacolon by delaying the clearance of the Shiga toxin. * **Infective Dose:** Shigella has a very low infective dose (as few as 10–100 organisms), making it highly contagious. * **Complication:** Watch for Hemolytic Uremic Syndrome (HUS), especially with *S. dysenteriae* type 1.

Question 1056: Which of the following drugs is most likely to cause additive anemia and neutropenia if administered to an AIDS patient taking zidovudine?

A. Acyclovir
B. Amantadine
C. Ganciclovir (Correct Answer)
D. Stavudine

Explanation: **Explanation:** The correct answer is **Ganciclovir**. **1. Why Ganciclovir is correct:** The primary dose-limiting toxicity of **Zidovudine (AZT)**, a Nucleoside Reverse Transcriptase Inhibitor (NRTI), is **bone marrow suppression**, leading to macrocytic anemia and neutropenia. **Ganciclovir**, used for CMV retinitis in AIDS patients, also causes significant myelosuppression (specifically neutropenia). When administered together, they exert a **synergistic/additive toxic effect** on the bone marrow, severely increasing the risk of life-threatening cytopenias. **2. Analysis of Incorrect Options:** * **Acyclovir:** While used for HSV/VZV in AIDS patients, it is primarily nephrotoxic (obstructive uropathy) rather than myelosuppressive. It does not significantly potentiate Zidovudine’s hematologic toxicity. * **Amantadine:** This is an anti-influenza A and anti-Parkinsonian drug. Its side effects are mainly CNS-related (insomnia, dizziness, *Livedo reticularis*) and it does not cause bone marrow suppression. * **Stavudine (d4T):** Although it is an NRTI like Zidovudine, the combination is avoided because they are **antagonistic**. Both require phosphorylation by the same enzyme (thymidine kinase); Zidovudine inhibits the phosphorylation of Stavudine, reducing its efficacy. Their major shared toxicity is peripheral neuropathy/lactic acidosis, not additive anemia. **Clinical Pearls for NEET-PG:** * **Zidovudine (AZT):** High-yield side effects include bone marrow suppression and nail hyperpigmentation. It is the drug of choice for preventing vertical transmission of HIV. * **Ganciclovir:** Most common side effect is neutropenia. If neutropenia becomes severe, **G-CSF (Filgrastim)** can be used. * **Other Myelosuppressive Drugs in AIDS:** Be cautious when combining Zidovudine with **Flucytosine, Sulfonamides (TMP-SMX), or Interferon-alpha**, as these also suppress the marrow.

Question 1057: Which antimicrobial agent inhibits ergosterol biosynthesis?

A. Ketoconazole (Correct Answer)
B. Amphotericin B
C. 5-flucytosine
D. Griseofulvin

Explanation: **Explanation:** The correct answer is **Ketoconazole**. To understand this, one must distinguish between drugs that inhibit the **synthesis** of ergosterol and those that bind to existing ergosterol. **1. Why Ketoconazole is correct:** Ketoconazole belongs to the **Azole** group of antifungals. Its primary mechanism of action is the inhibition of the enzyme **14-̑-demethylase** (a cytochrome P450 enzyme) [1]. This enzyme is responsible for converting lanosterol into ergosterol. By blocking this pathway, the fungal cell membrane becomes leaky and dysfunctional, leading to fungistatic effects [1]. **2. Why the other options are incorrect:** * **Amphotericin B:** This is a polyene antibiotic. It does not inhibit synthesis; instead, it **binds directly to ergosterol** already present in the fungal cell membrane, creating pores that cause leakage of intracellular ions (like $K^+$), leading to cell death [2], [3]. * **5-Flucytosine:** This is an antimetabolite. It is converted into 5-fluorouracil (5-FU) inside the fungal cell, which **inhibits DNA and RNA synthesis** [2], [4]. * **Griseofulvin:** This drug interferes with **mitotic spindle formation** by binding to polymerized microtubules, thereby inhibiting fungal mitosis. **Clinical Pearls for NEET-PG:** * **Squalene Epoxidase Inhibitors:** Terbinafine also inhibits ergosterol biosynthesis but at an earlier step (conversion of squalene to squalene epoxide). * **Echinocandins (e.g., Caspofungin):** These inhibit the synthesis of **̒-(1,3)-D-glucan**, a component of the fungal cell wall (not the membrane) [5]. * **Ketoconazole Side Effects:** It is notorious for inhibiting human steroid synthesis, leading to **gynecomastia** and decreased libido in males.

Question 1058: Which antimicrobial agent inhibits ergosterol biosynthesis?

A. Ketoconazole (Correct Answer)
B. Amphotericin B
C. 5-flucytosine
D. Griseofulvin

Explanation: ### Explanation **Correct Answer: A. Ketoconazole** **Mechanism of Action:** Ketoconazole belongs to the **Azole** class of antifungals. These agents work by inhibiting the fungal cytochrome P450 enzyme **14α-demethylase**. This enzyme is responsible for converting lanosterol into **ergosterol**, an essential component of the fungal cell membrane. Inhibition leads to the depletion of ergosterol and the accumulation of toxic methylated sterols, resulting in increased membrane permeability and fungal cell death. **Analysis of Incorrect Options:** * **B. Amphotericin B:** This is a polyene antibiotic. It does not inhibit the *synthesis* of ergosterol; instead, it **binds directly to pre-formed ergosterol** in the fungal cell membrane, creating pores that cause leakage of intracellular ions (like $K^+$). * **C. 5-Flucytosine:** This is an antimetabolite. It is converted into 5-fluorouracil (5-FU) inside the fungal cell, which **inhibits DNA and protein synthesis** by interfering with thymidylate synthase. * **D. Griseofulvin:** This agent **interferes with microtubule function**, thereby inhibiting mitosis (spindle formation) in dermatophytes. **High-Yield Clinical Pearls for NEET-PG:** * **Ketoconazole Side Effects:** It is a potent inhibitor of human CYP450 enzymes and steroidogenesis, leading to side effects like **gynecomastia**, loss of libido, and menstrual irregularities. * **Drug of Choice (DOC):** While Ketoconazole was the first oral azole, it has largely been replaced by **Itraconazole** (DOC for Sporotrichosis and Histoplasmosis) and **Fluconazole** (DOC for Candidiasis and Cryptococcal meningitis) due to better safety profiles. * **Terbinafine:** Another ergosterol synthesis inhibitor, but it acts earlier in the pathway by inhibiting **Squalene epoxidase**.

Question 1059: Mebendazole is used in the treatment of all EXCEPT:

A. Borrelia recurrentis (Correct Answer)
B. Giardia lamblia
C. Treponema vincenti
D. Gardnerella infections

Explanation: **Explanation:** The question asks for the exception regarding the clinical use of **Mebendazole**. However, there is a significant discrepancy in the provided options: Mebendazole is an **Anthelmintic** agent, while the options listed are bacteria and protozoa. In the context of the provided answer key, **Borrelia recurrentis** is the correct "exception" because it is a spirochete treated with Tetracyclines or Penicillin. However, it is crucial to note that Mebendazole is primarily used for **helminthic infections** (Nematodes like Ascaris, Hookworm, and Enterobius) by inhibiting microtubule synthesis [1], [3]. **Analysis of Options:** * **Borrelia recurrentis (Correct):** This is the causative agent of Relapsing Fever. It is a spirochete and is never treated with Mebendazole. The drug of choice is **Tetracycline** or **Erythromycin**. * **Giardia lamblia:** While Metronidazole is the drug of choice, Mebendazole and Albendazole have shown efficacy against Giardia by disrupting its cytoskeleton. * **Treponema vincenti & Gardnerella:** These are typically associated with anaerobic environments (e.g., Vincent’s angina or Bacterial Vaginosis). While Mebendazole is not a standard treatment, it belongs to the benzimidazole class, which shares structural similarities with **Nitroimidazoles** (like Metronidazole), leading to occasional cross-discussions in older pharmacological literature regarding anaerobic activity. **Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Mebendazole binds to **̢-tubulin**, inhibiting microtubule polymerization in helminths [3]. 2. **Spectrum:** It is highly effective against "Pinworm, Whipworm, Roundworm, and Hookworm" [1]. 3. **Pharmacokinetics:** It has poor systemic absorption, making it ideal for luminal (gut) parasites but less effective for systemic tissues compared to Albendazole [3]. 4. **Teratogenicity:** Benzimidazoles are generally avoided in the first trimester of pregnancy [2].

Question 1060: What is the drug of choice for treating cholera in pregnant women?

A. Tetracycline
B. Doxycycline
C. Furazolidone
D. None of the above (Correct Answer)

Explanation: The correct answer is **None of the above** because the current drug of choice (DOC) for cholera in pregnant women is **Azithromycin**. ### 1. Why "None of the above" is correct According to the WHO and current clinical guidelines, **Azithromycin** (a macrolide) is the preferred treatment for cholera in pregnant women and children. It is highly effective against *Vibrio cholerae*, achieves high concentrations in the intestinal lumen, and has a superior safety profile during pregnancy (FDA Category B). It is typically administered as a single 1g oral dose. ### 2. Why the other options are incorrect * **Tetracycline & Doxycycline (Options A & B):** While Doxycycline is the DOC for non-pregnant adults, tetracyclines are **contraindicated** in pregnancy. They cross the placenta and can cause permanent teeth discoloration (yellow-brown) and enamel hypoplasia in the fetus, as well as inhibit bone growth [1]. * **Furazolidone (Option C):** Historically, Furazolidone was used for cholera in pregnancy; however, it is no longer the first-line choice due to lower efficacy compared to macrolides and the availability of safer, more effective alternatives like Azithromycin. ### 3. Clinical Pearls for NEET-PG * **Primary Treatment:** The most critical intervention in cholera is **aggressive fluid resuscitation** (ORS or IV Ringer’s Lactate); antibiotics only shorten the duration of diarrhea and shedding. * **DOC Summary:** * **Adults (Non-pregnant):** Doxycycline (Single dose 300mg). * **Pregnant Women:** Azithromycin. * **Children:** Azithromycin is preferred (Zinc supplementation is also vital in pediatric cases). * **Resistance Note:** In areas with known macrolide resistance, **Ceftriaxone** may be considered as an alternative.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

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Tetracyclines

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Quinolones

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Sulfonamides and Trimethoprim

Practice Questions

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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