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?
Which of the following drugs is associated with untoward side effect of renal tubular damage?
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 :
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: Which of the following drugs is associated with untoward side effect of renal tubular damage?
- A. Streptozotocin
- B. Methysergide
- C. Cyclophosphamide
- D. Cisplatin (Correct Answer)
Explanation: ***Cisplatin*** - **Cisplatin** is a platinum-based chemotherapy drug well-known for its dose-limiting nephrotoxicity, primarily causing **renal tubular damage**. - Its mechanism involves direct DNA damage within renal tubular cells, leading to **acute tubular necrosis** if not managed with aggressive hydration and other protective measures. *Streptozotocin* - **Streptozotocin** is an alkylating agent primarily used in treating **pancreatic neuroendocrine tumors**; its main toxicity is typically to pancreatic beta cells (leading to hypoglycemia) and to the liver. - While it can be nephrotoxic, its predominant and most recognized untoward side effect is not renal tubular damage, but rather **pancreatic beta-cell destruction**. *Methysergide* - **Methysergide** is an ergot alkaloid used for **migraine prophylaxis** but is largely discontinued due to severe side effects like **retroperitoneal fibrosis**. - Renal damage in the context of methysergide is typically due to this fibrosis compressing the ureters, rather than direct tubular toxicity. *Cyclophosphamide* - **Cyclophosphamide** is an alkylating agent known for its immunosuppressive and chemotherapeutic effects; its major side effects include **hemorrhagic cystitis** and myelosuppression. - While high doses can cause nephrotoxicity, its primary and most feared renal-related toxicity is hemorrhagic cystitis, not direct tubular damage as seen with cisplatin.
Question 9: 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 10: 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.
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