Aminoglycosides — MCQs
Anaerobes are resistant intrinsically against which of the following?
Which of the following drugs does not inhibit protein synthesis?
A middle-aged man with chronic renal failure, diagnosed with sputum-positive pulmonary tuberculosis, has a creatinine clearance of 25 ml/min. All of the following drugs require modification in doses except:
Which of the following aminoglycosides is most cochleotoxic:-
Competitive neuromuscular blockade is enhanced by all except
Toxic focal myopathy is an adverse effect of:
Continued suppression of bacterial growth after antibiotic levels have fallen below the Minimum Inhibitory Concentration (MIC) is known as?
What is the mechanism of action of aminoglycoside antibiotics?
A 14-year-old boy presents with headache, fever, and cough for 2 days. Sputum is scant and non-purulent and gram stain reveals many white cells but no organisms. The treatment should be initiated with :
Which is the widest spectrum aminoglycoside?
Aminoglycosides Indian Medical PG Practice Questions and MCQs
Question 1: Anaerobes are resistant intrinsically against which of the following?
- A. Aminoglycosides (Correct Answer)
- B. Azithromycin
- C. Metronidazole
- D. Beta lactam antibiotics
Explanation: ***Aminoglycosides*** - **Aminoglycosides** require an **oxygen-dependent transport system** to enter bacterial cells. [3] - Since **anaerobes** thrive in low-oxygen environments, this transport system is inactive, making them intrinsically resistant to aminoglycosides. [3] *Azithromycin* - **Azithromycin** (a macrolide) inhibits protein synthesis by binding to the 50S ribosomal subunit. - Many anaerobes are susceptible to **azithromycin**, making it an effective treatment for certain anaerobic infections. *Metronidazole* - **Metronidazole** is a potent prodrug that requires reduction by **anaerobic metabolism** to become active. [1], [2] - Its mechanism of action involves creating **cytotoxic free radicals** that damage DNA, making it highly effective against most anaerobes. [2] *Beta lactam antibiotics* - **Beta-lactam antibiotics**, such as **penicillins** and **cephalosporins**, interfere with bacterial cell wall synthesis. - While some anaerobes are susceptible, others have developed resistance mechanisms like producing **beta-lactamase enzymes**, but they are not intrinsically resistant across the board. [4]
Question 2: Which of the following drugs does not inhibit protein synthesis?
- A. Beta-lactam antibiotics (Correct Answer)
- B. Tetracycline
- C. Chloramphenicol
- D. Erythromycin
Explanation: ***Beta-lactam antibiotics*** - **Beta-lactam antibiotics** (e.g., penicillin, cephalosporins) primarily inhibit bacterial **cell wall synthesis** by interfering with penicillin-binding proteins (PBPs) [1, 2]. - This mechanism is distinct from inhibiting protein synthesis, as they target the structural integrity of the bacterial cell rather than its metabolic machinery for protein production [1].*Tetracycline* - **Tetracyclines** bind to the **30S ribosomal subunit** of bacteria, preventing the attachment of aminoacyl-tRNA [3]. - This action directly inhibits the elongation of the polypeptide chain during protein synthesis, making it a protein synthesis inhibitor [3].*Chloramphenicol* - **Chloramphenicol** binds to the **50S ribosomal subunit**, specifically inhibiting the **peptidyl transferase** activity. - This prevents the formation of peptide bonds between amino acids, thereby blocking protein synthesis.*Erythromycin* - **Erythromycin**, a macrolide antibiotic, binds to the **50S ribosomal subunit** of bacteria. - It inhibits translocation, the process by which the ribosome moves along the mRNA, effectively halting protein synthesis.
Question 3: A middle-aged man with chronic renal failure, diagnosed with sputum-positive pulmonary tuberculosis, has a creatinine clearance of 25 ml/min. All of the following drugs require modification in doses except:
- A. Streptomycin
- B. Ethambutol
- C. Rifampicin (Correct Answer)
- D. Isoniazid
Explanation: ***Rifampicin*** - **Rifampicin** is primarily metabolized by the **liver** and excreted in bile, making dose adjustment largely unnecessary in renal impairment [1]. - Its elimination is minimally affected by **creatinine clearance** values as low as 25 ml/min, unlike extensively renally excreted drugs. *Isoniazid* - **Isoniazid** undergoes significant **renal excretion** of its active metabolites, requiring dose adjustment in severe renal impairment. - Accumulation of metabolites can lead to increased risk of **peripheral neuropathy** and other toxicities. *Streptomycin* - **Streptomycin** is almost entirely eliminated by the **kidneys** and is highly dependent on renal function. - In a patient with a creatinine clearance of 25 ml/min, the dose must be significantly reduced to avoid **ototoxicity** and **nephrotoxicity**. *Ethambutol* - **Ethambutol** is mainly eliminated by the **kidneys**, and its plasma levels can rise significantly with reduced renal function. - Dose adjustment is crucial to prevent dose-dependent **optic neuritis**, a severe side effect.
Question 4: Which of the following aminoglycosides is most cochleotoxic:-
- A. Streptomycin
- B. Amikacin
- C. Gentamycin (Correct Answer)
- D. Minocycline
Explanation: ***Gentamycin*** - **Gentamycin** is known to be the most **cochleotoxic** aminoglycoside, causing irreversible damage to the hair cells in the cochlea [1]. - This toxicity can lead to **permanent hearing loss** and **tinnitus** due to its selective accumulation in inner ear fluids [2]. *Streptomycin* - While streptomycin can cause ototoxicity, its primary adverse effect is vestibulo-toxicity, affecting **balance** more than hearing [2]. - It mainly targets the hair cells of the semicircular canals and otolithic organs, leading to **vertigo** and ataxia [3]. *Amikacin* - Amikacin is also an ototoxic aminoglycoside but is generally considered **less cochleotoxic** than gentamycin. - Its ototoxic effects are comparable to gentamicin, but it is often reserved for infections resistant to other aminoglycosides. *Minocycline* - Minocycline is a **tetracycline antibiotic**, not an aminoglycoside, and is not associated with significant ototoxicity. - Its side effects typically include photosensitivity, gastrointestinal upset, and **vestibular dysfunction** (dizziness, vertigo) in some patients, distinct from cochlear damage.
Question 5: Competitive neuromuscular blockade is enhanced by all except
- A. Acidosis
- B. Aminoglycosides
- C. Hypercalcemia (Correct Answer)
- D. Hypermagnesemia
Explanation: ***Hypercalcemia*** - **Hypercalcemia** generally **antagonizes** the effects of neuromuscular blockers by increasing acetylcholine release at the presynaptic terminal and sensitizing the postsynaptic membrane to acetylcholine. - Therefore, it would **reduce**, rather than enhance, competitive neuromuscular blockade. *Acidosis* - **Acidosis** enhances competitive neuromuscular blockade by altering the sensitivity of the **nicotinic acetylcholine receptors** at the neuromuscular junction. - It also affects the pharmacokinetics and pharmacodynamics of neuromuscular blocking agents, leading to **prolonged blockade**. *Aminoglycosides* - **Aminoglycoside antibiotics** like gentamicin or amikacin can enhance neuromuscular blockade by **inhibiting presynaptic acetylcholine release** and blocking postsynaptic nicotinic receptors. - This effect is particularly pronounced when given concurrently with competitive neuromuscular blocking agents. *Hypermagnesemia* - **Hypermagnesemia** enhances neuromuscular blockade by **decreasing acetylcholine release** from the presynaptic nerve terminal. - It also directly **depresses muscle fiber excitability** and reduces the sensitivity of the postsynaptic membrane to acetylcholine.
Question 6: Toxic focal myopathy is an adverse effect of:
- A. Penicillin
- B. Aminoglycosides
- C. Insulin
- D. Narcotics (Correct Answer)
Explanation: ***Narcotics*** - **Narcotics**, particularly **pentazocine**, when injected repeatedly into the same muscle group, can lead to **toxic focal myopathy**. - This condition involves localized muscle damage, necrosis, and woody fibrosis at the injection site due to the direct toxic effect of the drug on muscle tissue. - Pentazocine-induced myopathy is a well-documented adverse effect in patients with chronic intramuscular injections. *Penicillin* - **Penicillin** is generally associated with **hypersensitivity reactions** like rash, anaphylaxis, or interstitial nephritis. - It is not typically known to cause focal myopathy as a direct adverse effect. *Aminoglycosides* - **Aminoglycosides** are primarily associated with **nephrotoxicity** and **ototoxicity**. - They can also cause **neuromuscular blockade**, but not focal muscle damage through direct toxicity to muscle tissue. *Insulin* - **Insulin** administration can cause **lipohypertrophy** or **lipoatrophy** at injection sites due to fat tissue changes. - However, it does not directly lead to **toxic focal myopathy** involving muscle tissue damage.
Question 7: Continued suppression of bacterial growth after antibiotic levels have fallen below the Minimum Inhibitory Concentration (MIC) is known as?
- A. Time dependent killing
- B. Sequential blockade
- C. Concentration dependent killing
- D. Post antibiotic effect (Correct Answer)
Explanation: ***Post antibiotic effect*** - The **post-antibiotic effect (PAE)** refers to the continued suppression of bacterial growth after antibiotic levels have fallen below the **Minimum Inhibitory Concentration (MIC)**. - This phenomenon allows for less frequent dosing while maintaining efficacy, which is important for drug scheduling. *Time dependent killing* - **Time-dependent killing** means that the duration for which the antibiotic concentration stays above the **MIC** is the most important factor for efficacy, not necessarily the peak concentration. - Antibiotics with this characteristic, such as **beta-lactams**, often require frequent dosing or continuous infusion. *Sequential blockade* - **Sequential blockade** occurs when two drugs act on consecutive steps in a metabolic pathway, leading to a synergistic effect that results in enhanced microbial killing. - A classic example is the combination of **sulfamethoxazole and trimethoprim**, which inhibit different enzymes in the folic acid synthesis pathway. *Concentration dependent killing* - **Concentration-dependent killing** indicates that the rate and extent of bacterial killing increase as the antibiotic concentration rises, particularly when it exceeds the **MIC**. - Antibiotics like **aminoglycosides** exhibit this effect, often benefiting from high peak concentrations to maximize efficacy.
Question 8: What is the mechanism of action of aminoglycoside antibiotics?
- A. Inhibition of protein synthesis (Correct Answer)
- B. Inhibition of DNA replication
- C. Disruption of the cell membrane
- D. Inhibition of bacterial cell wall synthesis
Explanation: ***Inhibition of protein synthesis*** - Aminoglycosides **bind irreversibly to the 30S ribosomal subunit** of bacteria, interfering with the initiation complex formation and causing misreading of mRNA. - This leads to the production of **non-functional proteins** and ultimately bacterial cell death, making them bactericidal. *Disruption of the cell membrane* - This mechanism is characteristic of **polymyxins** (e.g., colistin), which interact with bacterial cell membranes, increasing permeability and causing leakage of intracellular contents. - Aminoglycosides do not primarily target the cell membrane for their bactericidal action. *Inhibition of DNA replication* - This mechanism is associated with **fluoroquinolones**, which inhibit bacterial topoisomerase II (DNA gyrase) and topoisomerase IV. - Aminoglycosides do not interfere with DNA synthesis or replication. *Inhibition of bacterial cell wall synthesis* - This is the mechanism of action for **beta-lactam antibiotics** (e.g., penicillins, cephalosporins) and **glycopeptides** (e.g., vancomycin), which target peptidoglycan synthesis. - Aminoglycosides do not affect the bacterial cell wall but rather their intracellular protein machinery.
Question 9: A 14-year-old boy presents with headache, fever, and cough for 2 days. Sputum is scant and non-purulent and gram stain reveals many white cells but no organisms. The treatment should be initiated with :
- A. Azithromycin (Correct Answer)
- B. Levofloxacin
- C. Amikacin
- D. Cefazolin
Explanation: ***Azithromycin*** - The clinical picture of **headache, fever, cough, scant non-purulent sputum**, and Gram stain showing white cells but no organisms is highly suggestive of **atypical pneumonia**, likely caused by *Mycoplasma pneumoniae* in this age group. - **Macrolides** like azithromycin are the **first-line treatment** for atypical pneumonia as they are effective against organisms like *Mycoplasma* and *Chlamydia* which lack cell walls and are therefore resistant to beta-lactam antibiotics. *Levofloxacin* - **Levofloxacin** is a **fluoroquinolone**, which is effective against atypical pathogens but is generally reserved for **older patients** or those with **allergies** to macrolides due to concerns about potential side effects like cartilage damage in children. - Using fluoroquinolones as a first-line treatment in adolescents for suspected atypical pneumonia is **not recommended** due to these potential side effects and the availability of safer alternatives. *Amikacin* - **Amikacin** is an **aminoglycoside antibiotic** primarily used for severe infections caused by **Gram-negative bacteria**. - It is **ineffective against atypical bacteria** like *Mycoplasma* or *Chlamydia* which are the likely causative agents in this scenario. *Cefazolin* - **Cefazolin** is a **first-generation cephalosporin**, which is a **beta-lactam antibiotic** effective mostly against Gram-positive cocci and some Gram-negative bacteria. - It is **ineffective against atypical pathogens** because these organisms **lack a cell wall** (like *Mycoplasma*) or have cell walls that are not targeted by beta-lactam antibiotics.
Question 10: Which is the widest spectrum aminoglycoside?
- A. Streptomycin
- B. Amikacin (Correct Answer)
- C. Framycetin
- D. Netilmicin
Explanation: ***Amikacin*** - **Amikacin** has the **widest spectrum** among aminoglycosides, showing activity against many gram-negative bacteria, including those resistant to other aminoglycosides. - Its chemical structure offers increased resistance to many **aminoglycoside-modifying enzymes**, broadening its utility against difficult-to-treat infections. *Streptomycin* - **Streptomycin** is primarily used for **tuberculosis** and certain other specific infections like plague and tularemia. - It has a much **narrower spectrum** compared to amikacin, with limited activity against many common gram-negative pathogens. *Framycetin* - **Framycetin** is mainly used as a **topical antibiotic** for skin, eye, and ear infections. - It has a **limited systemic use** due to its toxicity and spectrum, which is not as broad as amikacin. *Netilmicin* - **Netilmicin** offers activity against some gram-negative bacteria, including those resistant to gentamicin and tobramycin. - While it has a good spectrum, it is **not as broad** as amikacin, especially concerning strains with multiple resistance mechanisms.
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