Aminoglycoside antibiotics – classification and SAR MCQs With Answer

Aminoglycoside antibiotics – classification and SAR MCQs With Answer

Introduction: Aminoglycoside antibiotics are a vital class of bactericidal drugs used in B.Pharm practice for treating serious aerobic Gram-negative infections and selected Gram-positive organisms. This introduction reviews classification (streptidine vs 2-deoxystreptamine cores), structure–activity relationships (SAR) such as key amino groups, glycosidic linkages and side-chain modifications (e.g., amikacin’s L‑HABA), mechanisms (30S ribosomal binding, oxygen‑dependent uptake), resistance (AMEs, 16S rRNA methylation), clinical uses, toxicities (ototoxicity, nephrotoxicity) and therapeutic drug monitoring. Emphasis on SAR links chemical features to activity and resistance. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which cellular target do aminoglycoside antibiotics primarily bind to in bacteria?

• 50S ribosomal subunit
• 30S ribosomal subunit
• Bacterial DNA gyrase
• Peptidoglycan transpeptidase

Correct Answer: 30S ribosomal subunit

Q2. The antibacterial spectrum of aminoglycosides is best described as:

• Primarily anaerobic Gram-positive organisms
• Broad activity against fungi and viruses
• Excellent activity against aerobic Gram-negative bacilli
• Restricted to atypical intracellular organisms

Correct Answer: Excellent activity against aerobic Gram-negative bacilli

Q3. Why are aminoglycosides generally ineffective against strict anaerobes?

• They require oxygen-dependent active transport for uptake
• Anaerobes rapidly inactivate aminoglycosides enzymatically
• Aminoglycosides are inactivated by acidic pH in anaerobic sites
• Aminoglycosides only bind eukaryotic ribosomes

Correct Answer: They require oxygen-dependent active transport for uptake

Q4. Which structural core is common to gentamicin, tobramycin and amikacin?

• Streptidine nucleus
• 2‑deoxystreptamine (2‑DOS) core
• Macrolactone ring
• Anthracycline aglycone

Correct Answer: 2‑deoxystreptamine (2‑DOS) core

Q5. Amikacin resists many aminoglycoside‑modifying enzymes because of:

• A methylated 16S rRNA binding site
• An L‑hydroxyaminobutyric acid (L‑HABA) side chain
• A bicyclic streptidine core
• Its macrolide-like ring protecting glycosidic bonds

Correct Answer: An L‑hydroxyaminobutyric acid (L‑HABA) side chain

Q6. Which of the following is NOT a major enzyme family responsible for aminoglycoside resistance?

• AAC (acetyltransferases)
• ANT (nucleotidyltransferases/adenylyltransferases)
• APH (phosphotransferases)
• Beta-lactamases

Correct Answer: Beta-lactamases

Q7. In SAR terms, the antibacterial activity of aminoglycosides is closely related to:

• The number and position of positively charged amino groups
• The presence of an aromatic macrolide ring
• The molecular weight only, with larger always better
• Glycosylation with neutral sugars only

Correct Answer: The number and position of positively charged amino groups

Q8. The two most clinically important toxicities of aminoglycosides are:

• Hepatotoxicity and pancreatitis
• Ototoxicity and nephrotoxicity
• Agranulocytosis and thrombocytopenia
• Cardiotoxicity and pulmonary fibrosis

Correct Answer: Ototoxicity and nephrotoxicity

Q9. One rationale for once‑daily (extended‑interval) dosing of aminoglycosides is:

• They exhibit time-dependent killing requiring constant levels
• They have concentration-dependent killing and a post‑antibiotic effect
• They are rapidly metabolized by the liver
• Once‑daily dosing eliminates risk of ototoxicity completely

Correct Answer: They have concentration-dependent killing and a post‑antibiotic effect

Q10. High‑level resistance to aminoglycosides via target modification typically involves:

• Methylation of 16S rRNA preventing drug binding
• Alteration of the bacterial cell wall porins only
• Increased drug efflux pumps exclusive to Gram‑positives
• Cleavage of the drug by beta‑lactamases

Correct Answer: Methylation of 16S rRNA preventing drug binding

Q11. For aminoglycoside therapeutic drug monitoring, peak concentrations primarily correlate with:

• Toxicity risk
• Efficacy of bacterial killing
• Total daily urine output
• Hepatic clearance

Correct Answer: Efficacy of bacterial killing

Q12. Aminoglycosides show synergy with beta‑lactam antibiotics because:

• Beta‑lactams inhibit aminoglycoside metabolism
• Cell‑wall active drugs enhance aminoglycoside uptake into bacteria
• Both drugs bind the 30S ribosome together
• Beta‑lactams prevent renal excretion of aminoglycosides

Correct Answer: Cell‑wall active drugs enhance aminoglycoside uptake into bacteria

Q13. Which aminoglycoside is classically associated with vestibular ototoxicity and fetal deafness when used in pregnancy?

• Neomycin
• Streptomycin
• Tobramycin
• Amikacin

Correct Answer: Streptomycin

Q14. Aminoglycosides can exacerbate neuromuscular weakness and are therefore contraindicated in patients with:

• Diabetes mellitus
• Myasthenia gravis
• Hypertension
• Hypothyroidism

Correct Answer: Myasthenia gravis

Q15. Which aminoglycoside is commonly used topically or orally for bowel decontamination because it is poorly absorbed from the GI tract?

• Gentamicin
• Neomycin
• Streptomycin
• Amikacin

Correct Answer: Neomycin

Q16. Which aminoglycoside contains a streptidine nucleus rather than a 2‑deoxystreptamine core?

• Gentamicin
• Streptomycin
• Tobramycin
• Amikacin

Correct Answer: Streptomycin

Q17. The presence of a 2‑deoxystreptamine ring in aminoglycosides is important because it:

• Confers oral bioavailability
• Enhances binding to the 16S rRNA A‑site
• Prevents renal filtration
• Makes the drug lipophilic for intracellular penetration

Correct Answer: Enhances binding to the 16S rRNA A‑site

Q18. A key SAR modification that increases resistance to aminoglycoside‑modifying enzymes is:

• Removal of all amino groups
• Attachment of bulky side chains such as L‑HABA (amikacin)
• Conversion to a macrolide skeleton
• Replacing glycosidic bonds with peptide bonds

Correct Answer: Attachment of bulky side chains such as L‑HABA (amikacin)

Q19. AAC (aminoglycoside acetyltransferase) enzymes confer resistance by acetylating which functional group on the drug?

• Hydroxyl groups only
• Carboxyl groups
• Amino groups
• Thiol groups

Correct Answer: Amino groups

Q20. APH enzymes inactivate aminoglycosides by:

• Phosphorylating hydroxyl groups
• Acetylating amino groups
• Methylating ribosomal RNA
• Cleaving the glycosidic bond

Correct Answer: Phosphorylating hydroxyl groups

Q21. Which pregnant patient consideration is most important regarding aminoglycoside therapy?

• All aminoglycosides are completely safe in pregnancy
• Streptomycin is associated with fetal ototoxicity and should be avoided
• Aminoglycosides prevent congenital heart defects
• They are preferred drugs for urinary tract infections in pregnancy

Correct Answer: Streptomycin is associated with fetal ototoxicity and should be avoided

Q22. Which aminoglycoside is commonly administered by inhalation for chronic Pseudomonas infection in cystic fibrosis?

• Neomycin
• Tobramycin
• Streptomycin
• Amikacin

Correct Answer: Tobramycin

Q23. Which aminoglycoside is typically not absorbed orally and therefore used for hepatic encephalopathy or bowel decontamination?

• Gentamicin
• Neomycin
• Amikacin
• Tobramycin

Correct Answer: Neomycin

Q24. The bactericidal action of aminoglycosides involves incorporation of incorrect amino acids into proteins leading to:

• Stabilization of the 70S ribosome only
• Production of nonfunctional or toxic proteins and membrane damage
• Direct inhibition of DNA replication enzymes
• Inhibition of folate synthesis

Correct Answer: Production of nonfunctional or toxic proteins and membrane damage

Q25. How do the positive charges on aminoglycoside molecules contribute to their activity?

• They neutralize bacterial DNA so the drug is inactive
• They facilitate ionic interactions with the negatively charged rRNA backbone
• They make the drug lipid‑soluble for passive diffusion
• They enhance renal excretion only

Correct Answer: They facilitate ionic interactions with the negatively charged rRNA backbone

Q26. For empiric therapy against Pseudomonas aeruginosa, which aminoglycoside is often preferred due to potency?

• Gentamicin
• Neomycin
• Tobramycin
• Streptomycin

Correct Answer: Tobramycin

Q27. Uptake of aminoglycosides into Gram‑negative bacteria involves an initial binding to the outer membrane followed by:

• Diffusion through porins driven by passive gradient only
• Active energy‑dependent transport across the cytoplasmic membrane
• Direct endocytosis mediated by pili
• Export via efflux pumps into the periplasm

Correct Answer: Active energy‑dependent transport across the cytoplasmic membrane

Q28. Which laboratory parameter is most important to monitor during prolonged aminoglycoside therapy to detect nephrotoxicity?

• Serum transaminases
• Serum creatinine and creatinine clearance
• Serum amylase
• Platelet count

Correct Answer: Serum creatinine and creatinine clearance

Q29. Among commonly used aminoglycosides, which one generally has the broadest clinical spectrum and is most reserved for resistant infections?

• Gentamicin
• Tobramycin
• Amikacin
• Streptomycin

Correct Answer: Amikacin

Q30. The post‑antibiotic effect (PAE) observed with aminoglycosides is primarily due to:

• Rapid drug metabolism leaving inactive metabolites
• Persistent binding to ribosomes and delayed recovery of bacterial protein synthesis
• Immediate bacterial replication following drug removal
• Activation of bacterial stress response genes that protect against future drugs

Correct Answer: Persistent binding to ribosomes and delayed recovery of bacterial protein synthesis

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com