Macrolides are a clinically important class of antibiotics that includes erythromycin and related drugs such as clarithromycin and azithromycin. B. Pharm students must understand their mechanism of action (binding to the 23S rRNA of the 50S ribosomal subunit), antibacterial spectrum (effective against Gram‑positive cocci and atypical pathogens), pharmacokinetics (acid stability, tissue accumulation, CYP interactions), major adverse effects (gastrointestinal irritation, cholestatic hepatitis with estolate, QT prolongation) and resistance mechanisms (erm methylation, mef efflux). This concise guide and question set emphasize formulations, clinical uses, drug interactions and counseling points essential for safe, rational therapy with macrolides. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which is the primary mechanism of action of macrolides like erythromycin?
- Inhibition of cell wall synthesis by binding to transpeptidase
- Inhibition of DNA gyrase
- Binding to 23S rRNA of the 50S ribosomal subunit and blocking translocation
- Disruption of cytoplasmic membrane integrity
Correct Answer: Binding to 23S rRNA of the 50S ribosomal subunit and blocking translocation
Q2. Macrolides are generally classified as which type of antibacterial agents?
- Bactericidal against most pathogens
- Bacteriostatic, inhibiting protein synthesis
- Primarily fungistatic agents
- Only bactericidal against Gram-negative rods
Correct Answer: Bacteriostatic, inhibiting protein synthesis
Q3. Which macrolide is noted for a long half‑life and extensive tissue accumulation allowing once‑daily dosing?
- Erythromycin
- Clarithromycin
- Azithromycin
- Roxithromycin
Correct Answer: Azithromycin
Q4. Which macrolide is a potent inhibitor of cytochrome P450 3A4 and causes clinically significant drug interactions?
- Azithromycin
- Clarithromycin
- Fidaxomicin
- Spiramycin
Correct Answer: Clarithromycin
Q5. Which erythromycin formulation has been associated with cholestatic hepatitis and is generally avoided in susceptible patients?
- Erythromycin base
- Erythromycin estolate
- Erythromycin ethylsuccinate
- Erythromycin stearate
Correct Answer: Erythromycin estolate
Q6. Macrolides are especially effective against which group of pathogens?
- Most anaerobic Gram-negative bacilli only
- Atypical organisms such as Mycoplasma, Chlamydia and Legionella
- All Enterobacteriaceae uniformly
- Pseudomonas aeruginosa
Correct Answer: Atypical organisms such as Mycoplasma, Chlamydia and Legionella
Q7. Besides antibacterial effects, erythromycin can be used for which non‑antibiotic pharmacologic action?
- Anticoagulant effect via vitamin K antagonism
- Prokinetic agent via motilin receptor agonism
- Beta‑adrenergic bronchodilation
- Direct diuretic effect at the renal tubule
Correct Answer: Prokinetic agent via motilin receptor agonism
Q8. A common resistance mechanism to macrolides involves methylation of 23S rRNA. Which gene family encodes this mechanism?
- bla genes
- erm genes
- mecA genes
- tet genes
Correct Answer: erm genes
Q9. Macrolides exhibit cross‑resistance with which antibiotic class due to overlapping binding sites?
- Fluoroquinolones
- Aminoglycosides
- Lincosamides (e.g., clindamycin)
- Sulfonamides
Correct Answer: Lincosamides (e.g., clindamycin)
Q10. Which macrolide is commonly used as part of Helicobacter pylori eradication regimens?
- Erythromycin
- Clarithromycin
- Azithromycin
- Fidaxomicin
Correct Answer: Clarithromycin
Q11. A clinically important cardiovascular adverse effect of macrolides is:
- Hypertension due to sympathetic activation
- Torsades de pointes due to QT interval prolongation
- Atrial fibrillation induction
- Bradycardia due to AV block in all patients
Correct Answer: Torsades de pointes due to QT interval prolongation
Q12. Which macrolide has historically been considered relatively safe for use in pregnancy (excluding certain formulations)?
- Clarithromycin
- Azithromycin
- Erythromycin (non‑estolate salts)
- Roxithromycin
Correct Answer: Erythromycin (non‑estolate salts)
Q13. Which statement about erythromycin chemistry is correct?
- Erythromycin base is acid‑stable and well absorbed orally
- Erythromycin base is acid‑labile and may be inactivated in the stomach
- All erythromycin salts are equally hepatotoxic
- Erythromycin is a prodrug converted in plasma to clarithromycin
Correct Answer: Erythromycin base is acid‑labile and may be inactivated in the stomach
Q14. Macrolides bind to which bacterial ribosomal subunit?
- 30S ribosomal subunit
- 16S rRNA of 30S
- 50S ribosomal subunit
- 70S intact ribosome only
Correct Answer: 50S ribosomal subunit
Q15. Which macrolide dosing strategy is commonly used as a single‑dose therapy for uncomplicated Chlamydia trachomatis infection?
- Erythromycin 250 mg twice daily for 14 days
- Clarithromycin 500 mg twice daily for 7 days
- Azithromycin 1 g single oral dose
- Roxithromycin 300 mg single dose
Correct Answer: Azithromycin 1 g single oral dose
Q16. A common adverse effect associated with clarithromycin is:
- Renal crystalluria
- Metallic taste and dysgeusia
- Severe hyperglycemia
- Neutropenia as the most frequent event
Correct Answer: Metallic taste and dysgeusia
Q17. Co‑administration of erythromycin with which withdrawn prokinetic agent led to fatal cardiac arrhythmias, highlighting important drug interactions?
- Domperidone
- Cisapride
- Metoclopramide
- Bethanechol
Correct Answer: Cisapride
Q18. What is the primary route of elimination for most macrolides?
- Renal excretion as unchanged drug exclusively
- Hepatic metabolism and biliary excretion
- Exhalation via the lungs
- Renal tubular secretion via P‑glycoprotein only
Correct Answer: Hepatic metabolism and biliary excretion
Q19. For neonatal conjunctivitis prophylaxis, which macrolide formulation is commonly used as an ophthalmic ointment?
- Azithromycin oral suspension
- Erythromycin ophthalmic ointment
- Clarithromycin topical cream
- Roxithromycin eye drops
Correct Answer: Erythromycin ophthalmic ointment
Q20. Macrolides prevent which specific step in bacterial protein synthesis?
- Initiation complex formation by blocking fMet‑tRNA binding
- Peptidyl transferase activity at the 50S subunit
- Translocation of peptidyl‑tRNA from the A site to the P site
- Termination by inhibiting release factors
Correct Answer: Translocation of peptidyl‑tRNA from the A site to the P site
Q21. Which macrolide shows marked intracellular accumulation in phagocytes, aiding efficacy against intracellular pathogens?
- Erythromycin
- Clarithromycin
- Azithromycin
- Fidaxomicin
Correct Answer: Azithromycin
Q22. Which macrolide is least likely to cause clinically significant CYP3A4‑mediated drug interactions?
- Clarithromycin
- Erythromycin
- Azithromycin
- Telithromycin
Correct Answer: Azithromycin
Q23. Active efflux of macrolides is encoded by mef genes. What phenotype does this confer?
- High‑level methylation resistance to all macrolides and lincosamides
- Pump‑mediated resistance primarily reducing erythromycin susceptibility (M phenotype)
- Enzymatic hydrolysis of macrolides
- Target site modification by gyrase mutation
Correct Answer: Pump‑mediated resistance primarily reducing erythromycin susceptibility (M phenotype)
Q24. The most frequent tolerability problem with oral erythromycin is:
- Severe renal failure
- Gastrointestinal intolerance and cramping due to increased motility
- Severe neutropenia in most patients
- Acute pancreatitis as a common reaction
Correct Answer: Gastrointestinal intolerance and cramping due to increased motility
Q25. Use of erythromycin in young infants has been associated with which serious adverse event?
- Infantile hypertrophic pyloric stenosis
- Congenital cardiac malformations
- Permanent hearing loss in all infants
- Neonatal hypothyroidism
Correct Answer: Infantile hypertrophic pyloric stenosis
Q26. Combining a macrolide with clindamycin often results in which interaction at the bacterial level?
- Synergistic bactericidal activity
- No interaction, both act on separate targets
- Antagonism due to overlapping binding sites on the 50S subunit
- Enzymatic inactivation of clindamycin by macrolide
Correct Answer: Antagonism due to overlapping binding sites on the 50S subunit
Q27. A point mutation in which bacterial component commonly confers high‑level macrolide resistance?
- DNA gyrase subunit A
- 23S rRNA in the 50S ribosomal subunit
- 30S ribosomal protein S12
- Beta‑lactamase promoter region
Correct Answer: 23S rRNA in the 50S ribosomal subunit
Q28. Which macrolide is frequently recommended as part of outpatient therapy for community‑acquired atypical pneumonia?
- Azithromycin
- Fidaxomicin
- Linezolid
- Metronidazole
Correct Answer: Azithromycin
Q29. For topical treatment of acne vulgaris, which macrolide is commonly formulated as a topical preparation?
- Clarithromycin topical gel
- Erythromycin topical gel/solution
- Azithromycin cream
- Telithromycin lotion
Correct Answer: Erythromycin topical gel/solution
Q30. Which pharmacokinetic property of azithromycin largely explains its prolonged tissue concentrations after short dosing courses?
- Rapid renal clearance leading to high urinary levels
- High plasma protein binding and low Vd
- Large volume of distribution and intracellular accumulation in phagocytes
- Short elimination half‑life requiring multiple daily doses
Correct Answer: Large volume of distribution and intracellular accumulation in phagocytes
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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