Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1511: Multidrug-resistant (MDR) tuberculosis shows resistance to which of the following drugs?

A. Isoniazid, rifampicin, and fluoroquinolone
B. Fluoroquinolone
C. Isoniazid, rifampicin, and kanamycin
D. Isoniazid and rifampicin only (Correct Answer)

Explanation: ***Isoniazid and rifampicin only*** - **Multidrug-resistant (MDR) tuberculosis** is specifically defined by resistance to both **isoniazid** and **rifampicin**. - These two drugs are considered the most effective first-line anti-TB medications, making resistance to both a significant treatment challenge. *Isoniazid, rifampicin, and fluoroquinolone* - Resistance to **isoniazid**, **rifampicin**, and *any* fluoroquinolone defines **pre-extensively drug-resistant (pre-XDR) TB**, not MDR-TB. - Adding resistance to a fluoroquinolone indicates a more severe and harder-to-treat form of tuberculosis. *Fluoroquinolone* - Resistance to **fluoroquinolone** alone does not define MDR-TB; it is only one component of resistance that, when combined with resistance to isoniazid and rifampicin, signifies pre-XDR or XDR-TB. - While fluoroquinolones are important second-line drugs, their resistance in isolation does not meet the criteria for MDR-TB. *Isoniazid, rifampicin, and kanamycin* - Resistance to **isoniazid**, **rifampicin**, and *any* second-line injectable agent (like **kanamycin**, capreomycin, or amikacin) defines **extensively drug-resistant (XDR) TB**, not MDR-TB. - XDR-TB represents an even more complex and difficult form of the disease to treat, requiring highly specialized regimens.

Question 1512: Drug of choice for invasive aspergillosis is

A. Posaconazole
B. Voriconazole (Correct Answer)
C. Liposomal AMB
D. Caspofungin

Explanation: ***Voriconazole*** - **Voriconazole** is the **first-line drug of choice** for the treatment of **invasive aspergillosis** due to its superior efficacy and good CNS penetration compared to other antifungals. - It works by inhibiting the fungal enzyme **14-alpha-demethylase**, disrupting ergosterol synthesis in the fungal cell membrane. *Posaconazole* - **Posaconazole** is a broad-spectrum triazole antifungal, primarily used for **prophylaxis** of invasive fungal infections in high-risk patients. - While it has activity against *Aspergillus*, it is generally considered a **second-line agent** for treatment or for salvage therapy if voriconazole fails. *Liposomal AMB* - **Liposomal amphotericin B (AMB)** is an effective antifungal for invasive aspergillosis, particularly in patients who **cannot tolerate voriconazole** or have contraindications. - It is often considered a **second-line or alternative therapy** rather than the first-line drug of choice. *Caspofungin* - **Caspofungin** is an **echinocandin** antifungal that inhibits the synthesis of **beta-(1,3)-D-glucan**, a crucial component of the fungal cell wall. - While active against *Aspergillus*, it is typically used as **salvage therapy** or in combination with other antifungals for invasive aspergillosis, not as a monotherapy first-line drug.

Question 1513: What is the most appropriate antibiotic choice for a bedridden patient with catheter-related urinary tract infection (UTI) and pneumonia?

A. Amoxicillin
B. 3rd gen cephalosporins
C. 2nd gen cephalosporins
D. Piperacillin-tazobactam (Correct Answer)

Explanation: ***Piperacillin-tazobactam*** - This **broad-spectrum antibiotic** is effective against common uropathogens and respiratory pathogens, including **Gram-negative bacteria** (e.g., *Pseudomonas aeruginosa*) and **Gram-positive bacteria**, which are often implicated in **catheter-related UTIs** and **nosocomial pneumonia** in bedridden patients. - Its coverage of **anaerobes** also makes it suitable for mixed infections, and the **beta-lactamase inhibitor (tazobactam)** extends its spectrum against resistant strains. - Provides **comprehensive single-agent coverage** for both infection sites, making it ideal for empirical therapy in this critically ill patient. *Amoxicillin* - **Amoxicillin** is a narrow-spectrum penicillin primarily active against **Gram-positive bacteria** and some **Gram-negative bacteria**, but it lacks coverage against many common nosocomial pathogens and **beta-lactamase producing bacteria**. - It would be ineffective against resistant strains prevalent in **catheter-related UTIs** and hospital-acquired pneumonia. *3rd gen cephalosporins* - While **3rd generation cephalosporins** offer good coverage for many **Gram-negative bacteria**, they have limitations for this scenario: - **Ceftriaxone/cefotaxime**: Lack reliable activity against **_Pseudomonas aeruginosa_**, a critical pathogen in **catheter-related UTIs** and hospital-acquired pneumonia in bedridden patients. - **Ceftazidime**: Has anti-Pseudomonal activity but weaker **Gram-positive coverage** and no anaerobic coverage. - Neither provides the comprehensive empirical coverage needed for a severely ill patient with two concurrent serious nosocomial infections. *2nd gen cephalosporins* - **2nd generation cephalosporins** have a more limited spectrum compared to 3rd generation agents, with moderate activity against **Gram-negative bacteria** and some **Gram-positive organisms**. - They are typically not recommended for serious **nosocomial infections** like catheter-related UTIs or hospital-acquired pneumonia due to insufficient coverage against resistant pathogens and **_Pseudomonas aeruginosa_**.

Question 1514: Mechanism of action of Oseltamivir?

A. Neuraminidase inhibitor (Correct Answer)
B. Inhibitor of protein synthesis
C. Inhibitor of thymidylate synthetase
D. Pyrimidine analogs

Explanation: ***Neuraminidase inhibitor*** - **Oseltamivir** prevents the release of new influenza virus particles from infected cells by inhibiting **neuraminidase**, an enzyme essential for viral replication. - This inhibition limits the spread of the virus to uninfected cells, thereby reducing the severity and duration of the **flu**. *Inhibitor of protein synthesis* - This mechanism is characteristic of certain **antibiotics** (e.g., tetracyclines, macrolides) or **antitumor agents**, not antiviral medications like oseltamivir. - Inhibiting protein synthesis would interfere with the host cell's vital functions and is not the primary target for influenza antivirals. *Inhibitor of thymidylate synthetase* - **Thymidylate synthetase inhibitors** (e.g., 5-fluorouracil) are primarily used in **cancer chemotherapy** to block DNA synthesis in rapidly dividing cells. - This mechanism is not relevant to the action of oseltamivir against influenza virus. *Pyrimidine analogs* - **Pyrimidine analogs** (e.g., zidovudine, fluorouracil) are drugs that mimic natural pyrimidine bases and interfere with nucleic acid synthesis. - While some antivirals act as nucleoside/nucleotide analogs, this is a broader category, and oseltamivir's specific action is not as a pyrimidine analog.

Question 1515: Burkholderia cepacia is resistant to which of the following antibiotics?

A. Trimethoprim-sulfamethoxazole
B. Cefotetan (Correct Answer)
C. Ceftazidime
D. Temocillin

Explanation: ***Cefotetan*** - *Burkholderia cepacia* shows **consistent resistance** to second-generation cephalosporins and cephamycins like **cefotetan**. - This organism is intrinsically resistant to **aminoglycosides** (gentamicin, tobramycin) and **polymyxins** (colistin), and shows variable resistance to many beta-lactams. - Among the options provided, cefotetan represents the most consistently ineffective agent. *Ceftazidime* - **Ceftazidime** (third-generation cephalosporin) shows **variable susceptibility** with *B. cepacia*. - While resistance is common, it is **not uniform**, and ceftazidime is sometimes used in **combination therapy** for B. cepacia infections. - Not considered a classic example of intrinsic resistance. *Trimethoprim-sulfamethoxazole* - **TMP-SMX** is the **first-line treatment** for *Burkholderia cepacia* infections. - It demonstrates good activity and is the preferred antimicrobial agent for this organism. - Resistance can develop but is not intrinsic. *Temocillin* - **Temocillin** (carboxypenicillin) has demonstrated activity against *B. cepacia*. - Used particularly in Europe for treating infections caused by this organism. - Not an antibiotic to which *B. cepacia* shows consistent resistance.

Question 1516: What is the drug of choice for treating Cyclosporiasis?

A. Trimethoprim-sulfamethoxazole combination (Correct Answer)
B. Paromomycin
C. Metronidazole
D. Cyclosporin

Explanation: ***Trimethoprim-sulfamethoxazole combination*** - **Trimethoprim-sulfamethoxazole (TMP-SMX)** is the highly effective and **drug of choice** for treating **Cyclosporiasis**, particularly in both immunocompetent and immunocompromised individuals. - It is administered orally and works by inhibiting folic acid synthesis in the parasite. *Paromomycin* - **Paromomycin** is an aminoglycoside antibiotic primarily used for treating intestinal amebiasis and cryptosporidiosis, but it is **not effective** against Cyclosporiasis. - It exhibits poor systemic absorption and thus primarily acts within the gastrointestinal tract. *Metronidazole* - **Metronidazole** is commonly used to treat infections caused by anaerobic bacteria and certain parasites like *Giardia* and *Entamoeba histolytica*, but it is **ineffective** against *Cyclospora cayetanensis*. - Its mechanism of action involves disrupting DNA and protein synthesis in susceptible organisms. *Cyclosporin* - **Cyclosporin** is an immunosuppressant drug used to prevent organ transplant rejection and treat autoimmune diseases, and it has **no antiparasitic activity** against *Cyclospora*. - Its primary function is to inhibit T-lymphocyte activation.

Question 1517: What is the drug of choice for treating isosporiasis?

A. Penicillin G
B. Benzathine penicillin
C. Co-trimoxazole (Correct Answer)
D. Albendazole

Explanation: ***Co-trimoxazole*** - **Co-trimoxazole** (trimethoprim-sulfamethoxazole) is the **drug of choice** for treating **isosporiasis** (*Cystoisospora belli*, formerly *Isospora belli*), particularly in both immunocompetent and immunocompromised individuals. - Its mechanism of action involves inhibiting **folic acid synthesis** in the parasite through sequential blockade of the folate pathway, leading to its death. - Standard dose: **TMP 160 mg/SMX 800 mg QID for 10 days** in immunocompetent patients; HIV patients may require longer therapy and secondary prophylaxis. *Penicillin G* - **Penicillin G** is an **antibiotic** primarily used to treat bacterial infections, such as those caused by Streptococcus and Staphylococcus species. - It has **no activity** against **Isospora belli**, which is a parasitic protozoan infection. *Benzathine penicillin* - **Benzathine penicillin** is a **long-acting form of penicillin** commonly used for syphilis and rheumatic fever prevention due to its prolonged release. - Like penicillin G, it is an antibacterial agent and is **ineffective against parasitic infections** like isosporiasis. *Albendazole* - **Albendazole** is an **antihelminthic drug** used to treat a variety of helminth infections, such as hookworm, roundworm, and tapeworm. - It is **not effective** against **Isospora belli**, which is a protozoan parasite, not a helminth.

Question 1518: What is the primary site of action of amphotericin B?

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

Explanation: ***Plasma membrane*** - Amphotericin B primarily acts at the **fungal cell membrane**, specifically by binding to **ergosterol**, a sterol essential for fungal membrane integrity. - This binding leads to the formation of **pores** or channels in the membrane, causing leakage of intracellular ions and macromolecules, thus disrupting cellular function and causing cell death. *Cell wall* - The **cell wall** is mainly composed of **chitin** and glucans, which provide structural rigidity to fungal cells. - Amphotericin B does not directly target the fungal cell wall; instead, its action is focused on the underlying **plasma membrane**. *Protein* - Amphotericin B is a **polyene antifungal** medication and is not primarily involved in targeting or inhibiting protein synthesis. - Inhibition of **protein synthesis** is a mechanism seen in certain antibacterial agents (e.g., tetracyclines, macrolides) but not in amphotericin B. *Ribosomes* - **Ribosomes** are responsible for protein synthesis and are not the primary target of amphotericin B. - While disrupting the cell membrane can indirectly affect various cellular processes, including protein synthesis, it is not a direct site of action for this drug.

Question 1519: Which antiretroviral drug also has anti-hepatitis activity?

A. Abacavir
B. Nevirapine
C. Emtricitabine
D. Tenofovir (Correct Answer)

Explanation: ***Tenofovir*** - **Tenofovir** is a nucleotide reverse transcriptase inhibitor (NtRTI) that is highly effective against both **HIV** and **Hepatitis B virus (HBV)**. - It is a **first-line agent** for HBV treatment with a **high barrier to resistance**, making it the preferred choice for dual therapy in HIV/HBV coinfection. - It is FDA-approved for both HIV and HBV treatment and is a key component in antiretroviral therapy (ART) regimens for coinfected patients. *Abacavir* - **Abacavir** is a nucleoside reverse transcriptase inhibitor (NRTI) used for HIV treatment. - It does not have significant activity against **Hepatitis B virus (HBV)** or other hepatitis viruses. *Emtricitabine* - **Emtricitabine** is a nucleoside reverse transcriptase inhibitor (NRTI) that has activity against both **HIV** and **HBV**. - While it is FDA-approved for HBV treatment, it has a **lower barrier to resistance** compared to tenofovir and is less preferred as monotherapy for HBV. - It is commonly used in combination with tenofovir (as in Truvada) for enhanced efficacy in HIV/HBV coinfection. - **Tenofovir remains the best answer** due to its superior resistance profile and status as first-line HBV therapy. *Nevirapine* - **Nevirapine** is a non-nucleoside reverse transcriptase inhibitor (NNRTI) used specifically for **HIV infection**. - It does not demonstrate antiviral activity against **Hepatitis B virus (HBV)** or other forms of viral hepatitis.

Question 1520: Mycoplasma is inherently resistant to which of the following antibiotic classes due to its lack of a peptidoglycan cell wall?

A. Penicillins (e.g., Amoxicillin)
B. Ceftriaxone (a cephalosporin) (Correct Answer)
C. Aminoglycosides (e.g., Gentamicin)
D. Fluoroquinolones (e.g., Ciprofloxacin)

Explanation: ***Ceftriaxone (a cephalosporin)*** - Cephalosporins are **beta-lactam antibiotics** that inhibit **peptidoglycan cell wall synthesis** by binding to penicillin-binding proteins (PBPs) - *Mycoplasma* species **completely lack a peptidoglycan cell wall**, which is the primary target of all beta-lactam antibiotics - Therefore, cephalosporins (and all beta-lactams) are **inherently ineffective** against *Mycoplasma* infections - This is a classic example of **intrinsic resistance** due to absence of the drug target *Penicillins (e.g., Amoxicillin)* - While penicillins are also beta-lactam antibiotics that target cell wall synthesis, the question specifically asks about ceftriaxone - Penicillins are similarly ineffective against *Mycoplasma* for the same reason (lack of cell wall) - However, when given a specific drug example (ceftriaxone), it is the most precise answer *Aminoglycosides (e.g., Gentamicin)* - Aminoglycosides target the **30S ribosomal subunit** to inhibit **protein synthesis** - While they have **poor penetration** into *Mycoplasma* cells and limited clinical efficacy, the resistance is not due to lack of cell wall - The mechanism of reduced susceptibility is different from the intrinsic resistance seen with beta-lactams *Fluoroquinolones (e.g., Ciprofloxacin)* - Fluoroquinolones inhibit **DNA gyrase** and **topoisomerase IV**, enzymes essential for DNA replication - These antibiotics are **effective** against *Mycoplasma* species and are commonly used to treat *Mycoplasma* infections - They represent appropriate treatment options along with macrolides and tetracyclines

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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