Antimicrobial Agents — MCQs

A 45-year-old patient with a known allergy to penicillin presents with an enterococcal endocarditis. The physician needs to prescribe an antibiotic but wants to ensure it is safe for a penicillin allergy. The patient has had previous allergic reactions to penicillin including rash & swelling. Which of the following drugs can be used safely in a patient allergic to penicillin?

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1431: Which of the following drugs has gametocidal action against all species of Plasmodium?

A. Primaquine (Correct Answer)
B. Quinine
C. Chloroquine
D. None of the options

Explanation: ***Primaquine*** - **Primaquine** is a **8-aminoquinoline** that is effective against the **gametocytes** of all *Plasmodium* species, including *P. falciparum*. - Its gametocidal action is crucial for **blocking transmission** of malaria, as gametocytes are the parasite forms ingested by mosquitoes. *Quinine* - **Quinine** is a **blood schizonticide** primarily used for treating acute, uncomplicated malaria, especially due to **chloroquine-resistant *P. falciparum***. - While it has some activity against *P. vivax* and *P. malariae* gametocytes, its action against mature *P. falciparum* gametocytes is limited. *Chloroquine* - **Chloroquine** is a highly effective **blood schizonticide** for **sensitive *P. falciparum***, *P. vivax*, *P. ovale*, and *P. malariae*. - It rapidly clears asexual parasites but has no effect on **mature *P. falciparum* gametocytes** and therefore does not prevent transmission of this species. *None of the options* - This option is incorrect because **Primaquine** possesses the described broad-spectrum gametocidal activity.

Question 1432: Pneumococcal vaccine PPSV23 (23-valent polysaccharide) has shown the best results in the following people:

A. Cystic fibrosis
B. Recurrent otitis media and sinusitis
C. Sickle cell anemia (Correct Answer)
D. Child less than 2 years of age

Explanation: ***Sickle cell anemia*** - Patients with **sickle cell anemia** are at very high risk for **invasive pneumococcal disease** due to functional asplenia, making vaccination critical. - The **polysaccharide vaccine (PPSV23)** is recommended for individuals aged 2 years and older with chronic medical conditions like sickle cell disease as it elicits a T-cell-independent immune response. *Cystic fibrosis* - While individuals with **cystic fibrosis** are at increased risk for respiratory infections, the primary pathogens are typically *P. aeruginosa* and *S. aureus*, not *S. pneumoniae*. - Although pneumococcal vaccination is recommended for these patients, it does not show the "best results" in terms of reducing overall infection burden compared to those with **functional asplenia**. *Child less than 2 year age* - Children under **2 years of age** have an immature immune system that responds poorly to **polysaccharide vaccines** (like PPSV23) because they cannot mount a T-cell-independent response effectively. - The **conjugate vaccine (PCV13)**, which elicits a T-cell-dependent response, is therefore recommended for this age group. *Recurrent otitis media and sinusitis* - While *S. pneumoniae* is a common cause of **otitis media** and **sinusitis**, these conditions are typically non-invasive, and vaccination is a general recommendation rather than showing the "best results" in preventing severe, life-threatening outcomes. - The impact of **pneumococcal vaccination** in preventing recurrent otitis media and sinusitis in otherwise healthy individuals does not compare to its life-saving efficacy in immunosuppressed or asplenic patients.

Question 1433: What is the mechanism of resistance in MRSA?

A. PBP2a alteration (Correct Answer)
B. Efflux pump activation
C. Porins modification
D. Beta-lactamase production

Explanation: ***PBP2a alteration*** - Methicillin-resistant Staphylococcus aureus (MRSA) acquires the **mecA gene**, which encodes for a modified penicillin-binding protein, **PBP2a**. - **PBP2a** has a low affinity for **beta-lactam antibiotics**, allowing the bacteria to synthesize its cell wall even in the presence of these drugs. *Efflux pump activation* - Efflux pumps are mechanisms used by bacteria to actively pump out various antibiotics from their cells, leading to resistance. - While efflux pumps contribute to resistance against other antibiotics, they are **not the primary mechanism** of methicillin resistance in MRSA. *Porins modification* - Porins are channels in the outer membrane of Gram-negative bacteria that allow the passage of hydrophilic molecules, including some antibiotics. - Modification of porins is a common resistance mechanism in **Gram-negative bacteria** but is not relevant to MRSA, which is Gram-positive. *Beta-lactamase production* - Beta-lactamases are enzymes that **hydrolyze the beta-lactam ring** of antibiotics like penicillin, rendering them inactive. - While many Staphylococcus aureus strains produce beta-lactamase (penicillinase) causing resistance to penicillins, MRSA's resistance to methicillin and other broader-spectrum beta-lactams is primarily due to **PBP2a alteration**, not just beta-lactamase production.

Question 1434: Assertion: VZV vaccine is live attenuated. Reason: It cannot be given to immunocompromised patients.

A. Both true, reason doesn't explain assertion
B. Assertion true, reason false
C. Assertion false, reason true
D. Both true, reason explains assertion (Correct Answer)

Explanation: ***Both true, reason explains assertion*** - The **VZV (varicella-zoster virus) vaccine** is indeed a **live attenuated vaccine** containing weakened virus - the assertion is **TRUE** - It **cannot be given to immunocompromised patients** due to risk of vaccine-strain disease - the reason is **TRUE** - The reason **directly explains the assertion**: BECAUSE the vaccine is live attenuated, it poses infection risk and therefore cannot be used in immunocompromised individuals - The **causal relationship** is clear: live attenuated nature → contraindication in immunocompromised patients *Both true, reason doesn't explain assertion* - While both statements are factually true, this option would only be correct if the reason was unrelated to the assertion - However, the reason **directly explains WHY** the live attenuated nature is clinically significant - The contraindication is a **direct consequence** of the vaccine being live attenuated, so the reason does explain the assertion *Assertion true, reason false* - The assertion is true (VZV vaccine is live attenuated) - However, the reason is also **TRUE** - live attenuated vaccines are indeed contraindicated in immunocompromised patients due to risk of disseminated vaccine-strain infection - Since both statements are true, this option is incorrect *Assertion false, reason true* - The assertion is **TRUE**, not false - VZV vaccine (Varivax, Zostavax) is a **live attenuated vaccine** containing the Oka strain - This option incorrectly claims the assertion is false - Since the assertion is factually correct, this option cannot be right

Question 1435: Which antifungal binds to ergosterol, causing fungal cell membrane damage?

A. Caspofungin
B. Terbinafine
C. Fluconazole
D. Amphotericin B (Correct Answer)

Explanation: Amphotericin B - **Amphotericin B** is a polyene antifungal that directly binds to **ergosterol**, the primary sterol in fungal cell membranes [2]. - This binding creates pores in the membrane, leading to leakage of intracellular components and ultimately **fungal cell death** [2]. Caspofungin - **Caspofungin** is an echinocandin, which inhibits the synthesis of **β-(1,3)-D-glucan**, a vital component of the fungal cell wall. - Its mechanism of action is distinct from ergosterol binding and primarily targets cell wall integrity rather than the cell membrane directly. Terbinafine - **Terbinafine** is an allylamine antifungal that inhibits **squalene epoxidase**, an enzyme involved in ergosterol synthesis. - By blocking this enzyme, it prevents the formation of ergosterol, leading to an accumulation of toxic squalene and disrupting membrane function, but it does not directly bind to ergosterol. Fluconazole - **Fluconazole** is an azole antifungal that inhibits **lanosterol 14-α-demethylase**, a cytochrome P450 enzyme responsible for an earlier step in ergosterol synthesis [1]. - This action prevents ergosterol production, impairing membrane function, but it does not involve direct binding to pre-existing ergosterol [1].

Question 1436: Which antibiotic is best suited for Pseudomonas aeruginosa in a CF patient?

A. Ceftazidime (Correct Answer)
B. Clindamycin
C. Amoxicillin
D. Ciprofloxacin

Explanation: ***Ceftazidime*** - **Ceftazidime** is the **gold standard** first-line treatment for *Pseudomonas aeruginosa* in CF patients according to **CF Foundation guidelines**, often combined with an **aminoglycoside** like tobramycin. - It demonstrates **superior efficacy** against P. aeruginosa with lower propensity for **resistance development** compared to other antibiotics, making it the preferred choice for acute exacerbations. *Clindamycin* - **Clindamycin** is primarily effective against **gram-positive bacteria** and **anaerobes**; it has **no activity** against *Pseudomonas aeruginosa*. - It is commonly used for skin and soft tissue infections or anaerobic infections, but is not suitable for treating **gram-negative rods** like *P. aeruginosa*. *Amoxicillin* - **Amoxicillin** is a broad-spectrum penicillin effective against many **gram-positive** and some **gram-negative bacteria**, but it **completely lacks activity** against *Pseudomonas aeruginosa*. - Even with beta-lactamase inhibitors (amoxicillin-clavulanate), it has **no antipseudomonal coverage** and is not appropriate for CF-related Pseudomonas infections. *Ciprofloxacin* - While **ciprofloxacin** has good antipseudomonal activity and offers advantages like **oral bioavailability** and **lung penetration**, it is primarily used for **maintenance therapy** or mild outpatient cases. - It has concerns about **resistance development** when used as monotherapy and is not considered the **first-line choice** for acute P. aeruginosa infections in CF patients.

Question 1437: Dapsone is used for treatment of bacterial infections as well as for immunomodulatory actions. What is mechanism of dapsone for these indications?

A. Inhibition of cell wall synthesis
B. Inhibition of ergosterol in cell membranes
C. Competition with PABA in folic acid synthesis (Correct Answer)
D. Inhibition of protein synthesis

Explanation: ***Competition with PABA in folic acid synthesis*** - **Dapsone** is a **sulfone drug** that functions as a competitive antagonist of **para-aminobenzoic acid (PABA)**. - This competition blocks the enzyme **dihydropteroate synthase**, preventing **folic acid synthesis** in susceptible bacteria, which is essential for their growth. *Inhibition of cell wall synthesis* - This mechanism is characteristic of **beta-lactam antibiotics** (penicillins, cephalosporins) and **glycopeptides** (vancomycin), which target peptidoglycan synthesis. - **Dapsone** does not interfere with cell wall integrity or synthesis. *Inhibition of ergosterol in cell membranes* - This is the primary mechanism of action for many **antifungal drugs**, such as **azoles** (fluconazole) and **polyenes** (amphotericin B). - **Dapsone** does not target fungal cell membranes or ergosterol synthesis. *Inhibition of protein synthesis* - This mechanism is employed by various classes of **antibiotics**, including **tetracyclines**, **macrolides**, and **aminoglycosides**, which bind to bacterial ribosomes. - **Dapsone's** mechanism is distinct from inhibiting protein production.

Question 1438: Which of the following is the most commonly used drug for outpatient management of community acquired pneumonia?

A. Ceftriaxone
B. Streptomycin
C. Azithromycin (Correct Answer)
D. Vancomycin

Explanation: ***Azithromycin*** - **Azithromycin** is a macrolide antibiotic widely recommended for outpatient treatment of **community-acquired pneumonia (CAP)** due to its efficacy against common causative pathogens like *Streptococcus pneumoniae* and **atypical bacteria** (e.g., *Mycoplasma pneumoniae*, *Chlamydophila pneumoniae*). - Its **once-daily dosing** and good oral bioavailability contribute to patient compliance in an outpatient setting. *Ceftriaxone* - **Ceftriaxone** is a third-generation cephalosporin primarily used for **intravenous administration** in more severe CAP cases requiring hospitalization or for specific resistant strains, not typically for initial outpatient management. - While effective against *Streptococcus pneumoniae*, it lacks coverage for **atypical pathogens** and its parenteral route makes it less suitable for most outpatient settings. *Streptomycin* - **Streptomycin** is an aminoglycoside antibiotic primarily used for **tuberculosis** and severe gram-negative infections, not a standard treatment for typical or atypical pathogens causing CAP. - It has a high risk of **ototoxicity** and **nephrotoxicity**, making it unsuitable for routine outpatient CAP management. *Vancomycin* - **Vancomycin** is a glycopeptide antibiotic primarily used for **methicillin-resistant *Staphylococcus aureus* (MRSA)** infections and severe gram-positive infections, not a first-line agent for typical CAP pathogens. - It is usually administered **intravenously** and requires monitoring for **nephrotoxicity** and **ototoxicity**, making it inappropriate for routine outpatient use.

Question 1439: Which of the following anti-leprosy drugs can cause skin hyperpigmentation?

A. Clofazimine (Correct Answer)
B. Rifampicin
C. Ofloxacin
D. Dapsone

Explanation: ***Clofazimine*** - **Clofazimine** is known to cause dose-dependent **hyperpigmentation** of the skin, ranging from reddish-brown to dark brown, and can also discolor bodily fluids. - This side effect is due to the drug's accumulation in macrophages and subsequent deposition in the dermis and subcutaneous fat. *Rifampicin* - **Rifampicin** can cause harmless **reddish-orange discoloration** of urine, sweat, tears, and other bodily secretions. - It is not typically associated with **skin hyperpigmentation** (darkening of the skin itself). *Ofloxacin* - **Ofloxacin** is a fluoroquinolone antibiotic primarily used in leprosy treatment as a second-line agent. - Its common side effects include gastrointestinal disturbances and nervous system effects, but **skin hyperpigmentation** is not a characteristic adverse event. *Dapsone* - **Dapsone** is a sulfone drug and a cornerstone of leprosy treatment, but it is not associated with **skin hyperpigmentation**. - Its most notable side effects include **hemolysis**, especially in patients with **G6PD deficiency**, and methemoglobinemia.

Question 1440: A 45-year-old patient with a known allergy to penicillin presents with an enterococcal endocarditis. The physician needs to prescribe an antibiotic but wants to ensure it is safe for a penicillin allergy. The patient has had previous allergic reactions to penicillin including rash & swelling. Which of the following drugs can be used safely in a patient allergic to penicillin?

A. Ceftriaxone
B. Vancomycin (Correct Answer)
C. Piperacillin
D. Aztreonam

Explanation: ***Vancomycin*** - **Vancomycin** is a glycopeptide antibiotic with a completely different mechanism of action and chemical structure compared to penicillins, making it **safe for patients with penicillin allergy**. - It is the **first-line treatment for enterococcal endocarditis** in patients with penicillin allergy, providing excellent coverage against both *Enterococcus faecalis* and *E. faecium*. - No cross-reactivity with beta-lactam antibiotics. *Ceftriaxone* - **Ceftriaxone** is a third-generation **cephalosporin**, a beta-lactam antibiotic with structural similarity to penicillin. - There is **5-10% cross-reactivity risk** in patients with IgE-mediated penicillin allergy (rash, swelling, anaphylaxis), making it potentially unsafe in this patient. - Should be avoided in patients with a history of immediate hypersensitivity reactions to penicillin. *Piperacillin* - **Piperacillin** is a **penicillin derivative** (ureidopenicillin) and shares the same beta-lactam core structure that triggers allergic reactions. - **Absolutely contraindicated** in patients with known penicillin allergy—would almost certainly provoke an allergic reaction. - Using this drug would expose the patient to significant risk of anaphylaxis. *Aztreonam* - **Aztreonam** is a **monobactam** with minimal cross-reactivity (<1%) with penicillins and is generally **safe for penicillin-allergic patients** from an allergy perspective. - However, aztreonam has **primarily gram-negative coverage** and **poor activity against enterococci**, making it ineffective for treating enterococcal endocarditis. - While safe regarding allergy concerns, it is not the appropriate choice due to **lack of efficacy** against the causative organism.

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

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Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

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