Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1421: DOC for prophylaxis against Diphtheria is:-

A. Cloxacillin
B. Rifampicin
C. Ciprofloxacin
D. Erythromycin (Correct Answer)

Explanation: ***Erythromycin*** - **Erythromycin** is the drug of choice for prophylaxis and treatment of diphtheria, especially in individuals exposed to a confirmed case. - It works by inhibiting protein synthesis in *Corynebacterium diphtheriae*, preventing bacterial multiplication and toxin production. *Cloxacillin* - **Cloxacillin** is a narrow-spectrum penicillinase-resistant penicillin primarily used for treating staphylococcal infections. - It is not effective against *Corynebacterium diphtheriae* and therefore not used for diphtheria prophylaxis. *Rifampicin* - **Rifampicin** is an antibiotic commonly used for tuberculosis and prophylaxis against meningococcal disease. - It does not have significant activity against *Corynebacterium diphtheriae* and is not indicated for diphtheria prevention. *Ciprofloxacin* - **Ciprofloxacin** is a fluoroquinolone antibiotic with a broad spectrum of activity against many bacterial infections. - While it can be used for some respiratory infections, it is not the recommended antibiotic for diphtheria prophylaxis.

Question 1422: Which of the following is the first line drug for mastitis?

A. Cefazolin
B. Ampicillin
C. Metronidazole
D. Cloxacillin (Correct Answer)

Explanation: ***Cloxacillin*** - **Cloxacillin** is a penicillinase-resistant penicillin, making it effective against **Staphylococcus aureus**, the most common causative organism of mastitis. - It is the **first-line oral antibiotic** for treating mastitis in patients without penicillin allergy. - As a penicillin derivative, it should be **avoided in penicillin-allergic patients**, who may be treated with cephalosporins (if no cross-reactivity) or macrolides instead. *Cefazolin* - **Cefazolin** is a first-generation cephalosporin, typically administered intravenously. While effective against methicillin-sensitive *S. aureus*, it is not the first-line oral treatment for mastitis. - It is usually reserved for hospitalized patients or those needing parenteral therapy, not for initial outpatient management. *Ampicillin* - **Ampicillin** is a penicillin antibiotic that is susceptible to degradation by beta-lactamases produced by many *Staphylococcus aureus* strains. - It is generally **not effective** against beta-lactamase-producing *S. aureus*, which is the predominant pathogen in mastitis. *Metronidazole* - **Metronidazole** is an antibiotic primarily effective against **anaerobic bacteria** and certain **parasites**. - It has **no significant activity** against aerobic bacteria like *Staphylococcus aureus*, making it inappropriate for treating typical bacterial mastitis.

Question 1423: Agents most effective and best tolerated in the management of onychomycosis?

A. Ciclopirox and griseofulvin
B. Griseofulvin and ketoconazole
C. Griseofulvin and caspofungin
D. Itraconazole and terbinafine (Correct Answer)

Explanation: ***Itraconazole and terbinafine*** - **Terbinafine** is a highly effective **allylamine antifungal** for onychomycosis, achieving high cure rates with good tolerability due to its fungicidal action and accumulation in the nail plate. - **Itraconazole** is a **triazole antifungal** that is also effective for onychomycosis, particularly for both dermatophyte and non-dermatophyte infections, and can be given in pulse doses which may improve patient compliance and reduce side effects. *Ciclopirox and griseofulvin* - **Ciclopirox** is a topical agent, generally less effective for moderate to severe onychomycosis, and requires prolonged daily application. - **Griseofulvin** has a lower cure rate for onychomycosis compared to newer antifungals like terbinafine and itraconazole, and requires longer treatment durations with potential for more drug interactions. *Griseofulvin and ketoconazole* - **Griseofulvin** has been largely supplanted by newer antifungals due to its lower efficacy and longer treatment courses for onychomycosis. - **Ketoconazole** has a less favorable safety profile (e.g., hepatotoxicity) for systemic use in onychomycosis and is generally not recommended as a first-line treatment. *Griseofulvin and caspofungin* - **Griseofulvin** is an older oral antifungal with limited efficacy and prolonged treatment duration for onychomycosis. - **Caspofungin** is an **echinocandin** antifungal primarily used for systemic candidiasis and aspergillosis, and is not indicated for the treatment of onychomycosis.

Question 1424: In the treatment of granuloma inguinale, which antibiotic regimen is most appropriate?

A. Ceftriaxone 250mg IM single dose
B. Azithromycin single dose 1g
C. Doxycycline 100mg BD until lesions heal (Correct Answer)
D. Penicillin G 2.4 million units IM

Explanation: ***Doxycycline 100mg BD until lesions heal*** - **Doxycycline** is the recommended first-line treatment for **granuloma inguinale** (donovanosis) due to its effectiveness against **Klebsiella granulomatis**, the causative agent. - Treatment should continue until all lesions have completely healed, which may take at least 3 weeks or longer, to prevent relapse. *Ceftriaxone 250mg IM single dose* - This regimen is typically used for the treatment of **gonorrhea**, not granuloma inguinale. - While effective for some bacterial infections, it is not the appropriate therapy for **Klebsiella granulomatis**. *Azithromycin single dose 1g* - A single 1g dose of **azithromycin** is the standard treatment for **Chlamydia trachomatis** infections. - While azithromycin IS an alternative treatment for granuloma inguinale, it requires **1g weekly or 500mg daily** for at least 3 weeks, not a single dose. - The single-dose regimen shown here is therefore inappropriate for granuloma inguinale. *Penicillin G 2.4 million units IM* - This dosage of **penicillin G** is specifically used to treat **syphilis**, particularly early-stage syphilis. - It has no role in the treatment of granuloma inguinale.

Question 1425: In a case of recurrent genital herpes with >6 episodes per year, which prophylactic regimen is most appropriate?

A. Acyclovir 400mg BD
B. Famciclovir 250mg BD
C. Acyclovir 200mg 5 times daily
D. Valacyclovir 500mg daily (Correct Answer)

Explanation: ***Valacyclovir 500mg daily*** - **Valacyclovir 500mg once daily** is the most appropriate prophylactic regimen for frequent recurrent genital herpes (≥6 episodes/year) - **Once-daily dosing** significantly improves patient adherence compared to multiple daily doses, which is crucial for long-term suppressive therapy - **Higher bioavailability** than acyclovir (54% vs 15-20%), allowing for less frequent dosing - **CDC-recommended** suppressive therapy regimen with proven efficacy in reducing recurrence frequency by 70-80% - For patients with ≥10 episodes per year, the dose can be increased to **1g once daily** *Acyclovir 400mg BD* - This is a **valid alternative** CDC-recommended suppressive regimen - However, **twice-daily dosing** is less convenient for long-term prophylaxis compared to once-daily valacyclovir - **Lower bioavailability** requires more frequent dosing to maintain therapeutic levels - May be preferred in resource-limited settings due to lower cost *Famciclovir 250mg BD* - This is also a **valid CDC-recommended** suppressive therapy option - **Twice-daily dosing** reduces adherence compared to once-daily regimens - Generally considered a **second-line alternative** when acyclovir or valacyclovir are not tolerated - Effective but less commonly used as first-line suppressive therapy *Acyclovir 200mg 5 times daily* - This is an **acute episodic treatment** regimen for active outbreaks, not for suppressive prophylaxis - **Five times daily dosing** is impractical for long-term use and has poor adherence - Used for treating initial or recurrent episodes, typically for 5 days duration

Question 1426: A 65-year-old woman with recurrent herpes labialis develops erosions on hand and foot after starting systemic chemotherapy. Most appropriate antiviral prophylaxis is:

A. Oral acyclovir 400mg twice daily (Correct Answer)
B. Topical acyclovir only
C. Oral valacyclovir 500mg daily
D. Oral famciclovir 250mg daily

Explanation: ***Oral acyclovir 400mg twice daily*** - **Standard prophylactic regimen** for HSV suppression in **immunocompromised patients** undergoing chemotherapy - **Acyclovir 400mg BID** is well-established, cost-effective, and provides adequate systemic coverage to prevent recurrent HSV outbreaks and complications like eczema herpeticum - This dosing is recommended by major guidelines (CDC, IDSA) for **long-term suppressive therapy** in immunosuppressed individuals - The patient's erosions on hands and feet after starting chemotherapy indicate reactivation requiring systemic prophylaxis *Oral valacyclovir 500mg daily* - Valacyclovir has **superior bioavailability** compared to acyclovir, but **500mg once daily is insufficient** for immunocompromised patients - For chemotherapy patients, valacyclovir prophylaxis requires **500mg TWICE daily**, not once daily - Once-daily dosing (500mg) is appropriate only for **immunocompetent** patients with recurrent HSV *Topical acyclovir only* - **Topical antivirals** are inadequate for preventing systemic or widespread HSV reactivation in immunocompromised patients - The presentation of **erosions on hands and feet** suggests disseminated or eczema herpeticum, requiring **systemic therapy** - Topical agents have poor penetration and are only suitable for limited, localized mucocutaneous lesions *Oral famciclovir 250mg daily* - **250mg once daily is subtherapeutic** for HSV prophylaxis in immunocompromised patients - Standard suppressive famciclovir dosing is **250mg TWICE daily** or 500mg once daily for immunocompetent patients - For chemotherapy patients, higher or more frequent dosing would be needed, making this option incorrect

Question 1427: What is the mechanism of action of terbinafine?

A. Ergosterol synthesis inhibition
B. Cell wall synthesis inhibition
C. Squalene epoxidase inhibition (Correct Answer)
D. 14-α demethylase inhibition

Explanation: ***Squalene epoxidase inhibition***- Terbinafine is an allylamine antifungal that inhibits **squalene epoxidase**, an enzyme essential for **ergosterol biosynthesis** [1].- This inhibition leads to the accumulation of **squalene** within the fungal cell, which is toxic and disrupts cell membrane function.*Ergosterol synthesis inhibition*- While terbinafine ultimately inhibits ergosterol synthesis, its direct mechanism is not the inhibition of ergosterol itself but rather an enzyme involved in its pathway.- This option is too broad as other antifungals also inhibit ergosterol synthesis at different steps.*Cell wall synthesis inhibition*- This mechanism is characteristic of **echinocandins** (e.g., caspofungin), which inhibit **glucan synthase**, an enzyme involved in fungal cell wall formation.- Terbinafine does not target the fungal cell wall; its action is on the **cell membrane**.*14-α demethylase inhibition*- This is the primary mechanism of action for **azole antifungals** (e.g., fluconazole, itraconazole).- Inhibition of **14-α demethylase** also disrupts ergosterol synthesis by preventing the conversion of lanosterol to ergosterol.

Question 1428: Drug of choice for scrub typhus is:

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

Explanation: ***Doxycycline*** - **Doxycycline** is the drug of choice for **scrub typhus** in both adults and children due to its high efficacy. - It is a **tetracycline antibiotic** that inhibits bacterial protein synthesis, providing broad-spectrum coverage against *Orientia tsutsugamushi*. *Ciprofloxacin* - **Ciprofloxacin** is a **fluoroquinolone antibiotic** primarily used for bacterial infections, but it is **not effective** against *Orientia tsutsugamushi*. - It is generally reserved for specific conditions like urinary tract infections or some severe bacterial infections where other antibiotics are ineffective. *Azithromycin* - **Azithromycin** is a **macrolide antibiotic** that can be used as an alternative therapy for **scrub typhus**, especially in **pregnant women** and young children where doxycycline is contraindicated. - While effective, it is considered a **second-line agent** after doxycycline due to potentially slower resolution of symptoms. *Chloramphenicol* - **Chloramphenicol** was historically used for scrub typhus and is still effective, but its use is limited due to potential **severe side effects** like **bone marrow suppression** and **aplastic anemia**. - Due to these significant toxicities, it is generally reserved for cases where other safer and equally effective antibiotics are contraindicated or ineffective.

Question 1429: A patient on anti-tubercular drug therapy developed tingling sensation in the lower limbs. Which of the following drugs should be used for the treatment?

A. Pyridoxine (Correct Answer)
B. Thiamine
C. Folic acid
D. Vitamin B-12

Explanation: ***Pyridoxine*** - **Isoniazid**, a common anti-tubercular drug, can cause peripheral neuropathy due to its interference with **pyridoxine (vitamin B6)** metabolism [1]. Supplementation with **pyridoxine** is essential to prevent and treat this neuropathy, which manifests as a tingling sensation in the limbs [1]. *Thiamine* - **Thiamine (vitamin B1)** deficiency is associated with conditions like **beriberi** and **Wernicke-Korsakoff syndrome**, presenting with neurological and cardiac symptoms. It is not directly implicated in peripheral neuropathy caused by anti-tubercular drugs. *Folic acid* - **Folic acid** is important for DNA synthesis and red blood cell formation; deficiency can lead to **megaloblastic anemia** and some neurological symptoms. It does not directly address the mechanism of anti-tubercular drug-induced peripheral neuropathy. *Vitamin B-12* - **Vitamin B12 (cobalamin)** deficiency causes **pernicious anemia** and neurological symptoms like subacute combined degeneration of the spinal cord. While essential for nerve health, it is not the primary vitamin involved in preventing or reversing isoniazid-induced neuropathy.

Question 1430: Which of the following is a clinical use of tafenoquine?

A. Treatment of severe falciparum malaria
B. Prophylaxis of malaria in pregnancy
C. Treatment of endemic malaria in children < 2 years
D. Radical cure of Plasmodium vivax (Correct Answer)

Explanation: ***Radical cure of Plasmodium vivax*** - **Tafenoquine** is an 8-aminoquinoline derivative used for the **radical cure of *Plasmodium vivax*** malaria, targeting the **hypnozoites** in the liver. - It is effective in preventing relapse by clearing the dormant liver stages of the parasite. *Treatment of severe falciparum malaria* - **Severe falciparum malaria** is typically treated with intravenous **artesunate**, not tafenoquine. - Tafenoquine is not indicated for the acute treatment of severe, life-threatening malaria. *Prophylaxis of malaria in pregnancy* - **Tafenoquine** is **contraindicated in pregnancy** due to potential risks to the fetus and the mother, especially in case of **G6PD deficiency**. - **Mefloquine** and **chloroquine** are generally preferred for malaria prophylaxis in pregnant women, depending on local resistance patterns. *Treatment of endemic malaria in children < 2 years* - **Tafenoquine** is not approved for use in **children under 2 years of age** due to insufficient safety and efficacy data. - The risk of **hemolytic anemia** in children with undiagnosed G6PD deficiency adds to this contraindication.

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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