A 50-year-old man with normal renal function is being treated for Pseudomonas pyelonephritis. The unit gives the full daily gentamicin dose as a single infusion rather than in three divided doses, and his trough level is almost zero for several hours before the next dose. Why does the single daily dose remain effective?

Concentration-dependent killing with post-antibiotic effect

Time-dependent killing needing levels above MIC

Long half-life from slow renal elimination

Enterohepatic recycling that sustains levels

A 28-year-old man has a thigh abscess. Staphylococcus aureus from the pus is resistant to erythromycin and appears sensitive to clindamycin on disc testing. When the two discs are placed close together, the clindamycin zone is flattened on the side next to erythromycin. Which mechanism explains this result?

Methylation of 23S rRNA by an erm gene

Low-affinity PBP2a coded by mecA

Macrolide efflux coded by the msrA gene

Mutation in the DNA gyrase gene

A 30-year-old woman has fever of 39°C, vomiting, right flank pain and renal angle tenderness. Urine shows many pus cells. A clinic prescribes nitrofurantoin for 7 days. Why is this choice inappropriate?

It reaches therapeutic levels only in urine

Most E. coli strains are resistant to it

It is unsafe in women of child-bearing age

It acts only on gram-positive bacteria

A 55-year-old man on a ventilator in the ICU has pneumonia from carbapenem-resistant Acinetobacter baumannii, and IV colistin is started. Serum creatinine is checked daily. How does colistin kill the bacterium?

Disrupts the outer membrane by binding lipid A

Blocks the synthesis of mycolic acids

Blocks transpeptidation of peptidoglycan

Binds the 30S subunit and causes misreading

Antibacterials by class and their adverse effects

29 NEET-PG practice questions on Antibacterials by class and their adverse effects, part of Pharmacology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 60-year-old man develops a wound infection 5 days after hip replacement in a Pune hospital. Culture grows Staphylococcus aureus resistant to cefoxitin (disc diffusion) and oxacillin. The resident writes four statements: 1. Resistance is due to PBP2a, coded by the mecA gene. 2. Vancomycin acts by binding the D-Ala-D-Ala end of cell wall precursors. 3. Amoxicillin-clavulanate will work because clavulanate inactivates the resistance enzyme. 4. Linezolid, an alternative, can cause thrombocytopenia when given for more than 2 weeks. Which statements are correct?
1. A1, 2 and 3 only
2. B1, 2 and 4 only
3. C2, 3 and 4 only
4. D1 and 2 only
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Answer: B · 1, 2 and 4 only
Statements 1, 2 and 4 are correct. MRSA carries mecA, which codes for PBP2a, a transpeptidase with low affinity for beta-lactams; cefoxitin is the laboratory marker because it induces mecA well. Vancomycin binds the D-Ala-D-Ala terminus of peptidoglycan precursors and blocks their incorporation, and linezolid, which binds the 50S ribosomal subunit, causes reversible myelosuppression, mainly thrombocytopenia, with prolonged courses. Statement 3 is false: the resistance is not due to a beta-lactamase, so adding clavulanate does not help; ceftaroline is a beta-lactam that does bind PBP2a.
- A: Includes statement 3; a beta-lactamase inhibitor cannot overcome an altered PBP.
- C: Includes statement 3 and omits statement 1, the actual mechanism of MRSA.
- D: Omits the linezolid adverse effect in statement 4, which is correct.
Question 2 · Medium
A 50-year-old man with normal renal function is being treated for Pseudomonas pyelonephritis. The unit gives the full daily gentamicin dose as a single infusion rather than in three divided doses, and his trough level is almost zero for several hours before the next dose. Why does the single daily dose remain effective?
1. ATime-dependent killing needing levels above MIC
2. BLong half-life from slow renal elimination
3. CEnterohepatic recycling that sustains levels
4. DConcentration-dependent killing with post-antibiotic effect
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Answer: D · Concentration-dependent killing with post-antibiotic effect
Aminoglycosides kill in proportion to their peak concentration relative to the MIC, and they suppress bacterial growth for hours after levels fall below the MIC (the post-antibiotic effect). A high once-daily peak therefore kills well, while the long drug-free trough lets the saturable uptake into renal tubular and inner ear cells settle, which reduces toxicity. Once-daily dosing is avoided or modified in endocarditis, pregnancy and significant renal impairment. Trough levels are checked to make sure the drug is not accumulating.
- A: Time-dependent killing describes beta-lactams, which need levels above the MIC for most of the interval.
- B: Gentamicin's half-life is short, about 2-3 hours, with normal kidneys.
- C: Aminoglycosides are excreted unchanged by the kidney, not recycled through bile.
Question 3 · Medium
A 28-year-old man has a thigh abscess. Staphylococcus aureus from the pus is resistant to erythromycin and appears sensitive to clindamycin on disc testing. When the two discs are placed close together, the clindamycin zone is flattened on the side next to erythromycin. Which mechanism explains this result?
1. ALow-affinity PBP2a coded by mecA
2. BMacrolide efflux coded by the msrA gene
3. CMethylation of 23S rRNA by an erm gene
4. DMutation in the DNA gyrase gene
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Answer: C · Methylation of 23S rRNA by an erm gene
A positive D-test shows inducible clindamycin resistance. Erythromycin induces an erm methylase that methylates the 23S rRNA of the 50S subunit, the shared binding site of macrolides, lincosamides and streptogramin B (MLSB resistance). Clindamycin may therefore fail during treatment despite looking sensitive, and it should be reported as resistant. Efflux by msrA causes erythromycin resistance with a negative D-test, and clindamycin remains usable.
- A: PBP2a causes methicillin resistance and does not affect clindamycin.
- B: msrA efflux spares clindamycin and gives a round, unflattened zone.
- D: Gyrase mutations cause fluoroquinolone resistance.
Question 4 · Medium
A 30-year-old woman has fever of 39°C, vomiting, right flank pain and renal angle tenderness. Urine shows many pus cells. A clinic prescribes nitrofurantoin for 7 days. Why is this choice inappropriate?
1. AMost E. coli strains are resistant to it
2. BIt is unsafe in women of child-bearing age
3. CIt acts only on gram-positive bacteria
4. DIt reaches therapeutic levels only in urine
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Answer: D · It reaches therapeutic levels only in urine
This is acute pyelonephritis. Nitrofurantoin is rapidly excreted and concentrated in urine, but its levels in blood and kidney tissue are too low to treat upper urinary tract or systemic infection. It is therefore used only for uncomplicated cystitis and prophylaxis. Pyelonephritis needs a drug with good tissue levels, such as a cephalosporin or a fluoroquinolone, guided by urine culture.
- A: Most community E. coli remain susceptible to nitrofurantoin.
- B: It is widely used in women, including in pregnancy except near term.
- C: It is active against E. coli and other gram-negative urinary pathogens.
Question 5 · Medium
A 55-year-old man on a ventilator in the ICU has pneumonia from carbapenem-resistant Acinetobacter baumannii, and IV colistin is started. Serum creatinine is checked daily. How does colistin kill the bacterium?
1. ABlocks the synthesis of mycolic acids
2. BDisrupts the outer membrane by binding lipid A
3. CBlocks transpeptidation of peptidoglycan
4. DBinds the 30S subunit and causes misreading
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Answer: B · Disrupts the outer membrane by binding lipid A
Colistin (polymyxin E) is a cationic peptide that binds the negatively charged lipid A of lipopolysaccharide, displaces calcium and magnesium, and disrupts the outer and inner membranes like a detergent. It is active only against gram-negative bacteria and is a last-resort drug for carbapenem-resistant Acinetobacter, Pseudomonas and Enterobacterales. Its main toxicities are nephrotoxicity and neurotoxicity, including paraesthesia and neuromuscular blockade, so renal function is monitored. Resistance can arise from modification of lipid A, including the plasmid gene mcr-1.
- A: Mycolic acid synthesis is the target of isoniazid.
- C: Transpeptidation is blocked by beta-lactams, which this strain resists.
- D: Misreading from 30S binding is the action of aminoglycosides.
Question 6 · Hard
A 70-year-old man with Enterococcus faecalis endocarditis is treated with IV ampicillin and gentamicin. He also takes furosemide for heart failure. In week 2 his creatinine rises from 1.0 to 1.8 mg/dL and the gentamicin trough level is high. The registrar notes: 1. Ampicillin damages the cell wall and improves gentamicin entry into the bacteria. 2. Furosemide adds to the risk of ototoxicity. 3. Aminoglycoside nephrotoxicity is usually permanent, while ototoxicity usually recovers. 4. A rising trough level shows that gentamicin is accumulating. Which notes are correct?
1. A1, 2 and 4 only
2. B1 and 3 only
3. C1 and 2 only
4. D3 and 4 only
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Question 7 · Easy
A 26-year-old woman at 20 weeks of pregnancy comes to the antenatal OPD with burning micturition. Urine culture grows more than 10^5 CFU/mL of E. coli, sensitive to every agent tested. Which oral antibiotic is the most appropriate?
1. ACefalexin
2. BChloramphenicol
3. CDoxycycline
4. DLevofloxacin
Answer and explanation in LearnFree account
Question 8 · Medium
A 60-year-old man in the ICU has a central line infection. Blood culture grows Enterococcus faecium resistant to both vancomycin and teicoplanin. Which change explains this resistance?
1. APrecursors ending in D-Ala-D-Lac
2. BEfflux of vancomycin from the cell
3. CA low-affinity PBP5
4. DA thickened, drug-trapping cell wall
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Question 9 · Hard
A 24-year-old woman at 16 weeks of pregnancy has a VDRL titre of 1:32 and a positive TPHA in the antenatal clinic. She had anaphylaxis after a penicillin injection three years ago. What is the best next step in treatment?
1. ASingle-dose oral azithromycin
2. BDesensitise, then benzathine penicillin
3. CIM ceftriaxone for 10 days
4. DOral doxycycline for 14 days
Answer and explanation in LearnFree account
Question 10 · Medium
A 68-year-old man with COPD on sustained-release theophylline is given ciprofloxacin for a urinary infection. On day 3 he has palpitations, vomiting and a generalised seizure. His serum theophylline level is 32 µg/mL (target 10-20 µg/mL). What explains the raised level?
1. AInhibition of CYP1A2 by ciprofloxacin
2. BInduction of CYP3A4 by ciprofloxacin
3. CReduced renal excretion of theophylline
4. DDisplacement from albumin by ciprofloxacin
Answer and explanation in LearnFree account
Question 11 · Medium
A 3-year-old girl with acute otitis media still has fever and a bulging red eardrum after 3 days of high-dose amoxicillin. Fluid from tympanocentesis grows non-typeable Haemophilus influenzae that produces beta-lactamase. Which antibiotic is the most appropriate change?
1. AHigher-dose amoxicillin
2. BOral erythromycin
3. COral cloxacillin
4. DAmoxicillin-clavulanate
Answer and explanation in LearnFree account
Question 12 · Medium
A 64-year-old diabetic woman with an indwelling catheter has urosepsis with BP 88/56 mmHg. Blood culture grows Klebsiella pneumoniae that produces an extended-spectrum beta-lactamase; it is resistant to ceftriaxone and sensitive to meropenem. Which drug is the most reliable choice?
1. ACeftriaxone
2. BAmoxicillin-clavulanate
3. CMeropenem
4. DCefepime
Answer and explanation in LearnFree account
Question 13 · Easy
A 74-year-old man on long-term prednisolone for COPD is given levofloxacin for a chest infection. On day 5 he feels a sudden snap at the back of his left heel while climbing stairs and cannot stand on tiptoe on that side. Calf squeeze does not move the foot. What has most likely happened?
1. ADeep vein thrombosis
2. BAchilles tendon rupture
3. CCalf muscle haematoma
4. DAcute gouty arthritis
Answer and explanation in LearnFree account
Question 14 · Medium
A 6-day-old term neonate is admitted with suspected sepsis. Serum bilirubin is 16 mg/dL. The resident writes for IV ceftriaxone, but the consultant changes it to cefotaxime. What is the reason for the change?
1. ACeftriaxone displaces bilirubin from albumin
2. BCeftriaxone misses group B streptococci
3. CCefotaxime enters the CSF much better
4. DCeftriaxone causes grey baby syndrome
Answer and explanation in LearnFree account
Question 15 · Hard
A 66-year-old man on simvastatin 40 mg and amlodipine is given a 14-day H. pylori regimen of clarithromycin, amoxicillin and a proton pump inhibitor for a duodenal ulcer. On day 6 he has severe muscle pain and weakness and passes dark urine. CK is 18,000 U/L and urine dipstick is positive for blood with no red cells on microscopy. What is the mechanism of this reaction?
1. ACYP3A4 induction by clarithromycin
2. BDirect muscle toxicity of amoxicillin
3. CCYP3A4 inhibition by clarithromycin
4. DCYP2C9 inhibition by clarithromycin
Answer and explanation in LearnFree account
Question 16 · Medium
A 22-year-old student in Delhi with 8 days of fever has Salmonella Typhi in a blood culture. The isolate is resistant to nalidixic acid and has a raised ciprofloxacin MIC, and his fever settled only slowly on ciprofloxacin. What is the most likely mechanism of this reduced susceptibility?
1. AAltered dihydropteroate synthase
2. BMutation in the DNA gyrase gene gyrA
3. CMethylation of the 23S ribosomal RNA
4. DAn acquired extended-spectrum beta-lactamase
Answer and explanation in LearnFree account
Question 17 · Easy
A 72-year-old woman given oral clindamycin after drainage of a dental abscess develops 8 watery stools a day, fever and a TLC of 14,000/µL on day 6. Stool is positive for Clostridioides difficile toxin. Clindamycin is stopped. Which treatment is the most appropriate?
1. AIntravenous vancomycin
2. BOral vancomycin
3. COral loperamide
4. DOral ciprofloxacin
Answer and explanation in LearnFree account
Question 18 · Medium
A 30-year-old man with HIV is treated for Pneumocystis pneumonia with high-dose IV cotrimoxazole. On day 5 his serum potassium is 6.4 mmol/L; serum creatinine and blood glucose are normal and he takes no other drugs. How does the drug raise potassium?
1. AInhibiting aldosterone synthesis
2. BBlocking tubular creatinine secretion
3. CCausing intravascular haemolysis
4. DBlocking epithelial sodium channels
Answer and explanation in LearnFree account
Question 19 · Hard
A 54-year-old woman with type 2 diabetes on glimepiride, who takes a calcium-magnesium antacid for dyspepsia, is started on oral levofloxacin for community-acquired pneumonia. She is also on azithromycin from another clinic. Consider four statements: 1. Levofloxacin inhibits bacterial DNA gyrase and topoisomerase IV. 2. Taking it with her antacid improves its absorption. 3. It may cause hypoglycaemia in her while she takes glimepiride. 4. Her ECG should be reviewed, as both antibiotics can prolong the QT interval. Which statements are correct?
1. A1, 2, 3 and 4
2. B1 and 2 only
3. C1 and 3 only
4. D1, 3 and 4 only
Answer and explanation in LearnFree account
Question 20 · Medium
A 45-year-old man has a laparotomy under general anaesthesia with vecuronium, which is reversed at the end of surgery. In the recovery room he receives a large IV bolus of gentamicin. Fifteen minutes later he becomes weak, his breathing turns shallow and SpO2 falls to 88%. Which drug most specifically counters this effect of the antibiotic?
1. ANaloxone
2. BCalcium gluconate
3. CFlumazenil
4. DDantrolene
Answer and explanation in LearnFree account
Question 21 · Medium
A 24-year-old man with secondary syphilis (rash on palms and soles, VDRL 1:64) receives benzathine penicillin 2.4 million units IM. Four hours later he has fever of 39°C, rigors, headache and myalgia, and the rash looks brighter. BP is 118/76 mmHg and his chest is clear. What is the most appropriate management?
1. ALabel penicillin allergy; use doxycycline
2. BIM adrenaline for anaphylaxis
3. CAntipyretics and reassurance
4. DIV hydrocortisone for serum sickness
Answer and explanation in LearnFree account
Question 22 · Hard
A 72-year-old man on long-term prednisolone for rheumatoid arthritis is admitted with fever, headache, neck stiffness and confusion. CSF shows 650 cells/µL, mostly neutrophils, protein 160 mg/dL and glucose 30 mg/dL (blood 110 mg/dL). Gram stain shows a few short gram-positive rods. IV ceftriaxone and vancomycin have been started. What change is needed?
1. AAdd IV ampicillin
2. BDouble the ceftriaxone dose
3. CAdd IV azithromycin
4. DAdd IV metronidazole
Answer and explanation in LearnFree account
Question 23 · Easy
A 40-year-old man with MRSA bacteraemia is given IV vancomycin 1 g run in over 15 minutes. Near the end of the infusion he develops flushing and an itchy red rash over his face, neck and upper chest, with mild hypotension. There is no wheeze or stridor. How should the next dose be given?
1. AReplaced permanently by linezolid
2. BInfused slowly over at least 1 hour
3. CPreceded by IM adrenaline
4. DGiven only after desensitisation
Answer and explanation in LearnFree account
Question 24 · Medium
A 45-year-old woman on sertraline for depression is started on IV linezolid for a complicated MRSA skin infection. On day 2 she is agitated and tremulous, with sweating, temperature 38.8°C, pulse 124/min, hyperreflexia and inducible ankle clonus. How did linezolid contribute to this reaction?
1. AInhibition of mitochondrial ribosomes
2. BInhibition of monoamine oxidase
3. CInhibition of hepatic CYP2D6
4. DBlockade of dopamine D2 receptors
Answer and explanation in LearnFree account
Question 25 · Medium
A 5-year-old boy in Pune has pneumococcal meningitis. The CSF isolate of Streptococcus pneumoniae has a penicillin MIC of 2 µg/mL and is beta-lactamase negative. What is the mechanism of its penicillin resistance?
1. AActive efflux of the drug
2. BLoss of outer membrane porins
3. CAltered penicillin-binding proteins
4. DProduction of a beta-lactamase
Answer and explanation in LearnFree account

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Antibacterials by class and their adverse effects

A 30-year-old woman has fever of 39°C, vomiting, right flank pain and renal angle tenderness. Urine shows many pus cells. A clinic prescribes nitrofurantoin for 7 days. Why is this choice inappropriate?

A 55-year-old man on a ventilator in the ICU has pneumonia from carbapenem-resistant Acinetobacter baumannii, and IV colistin is started. Serum creatinine is checked daily. How does colistin kill the bacterium?

A 26-year-old woman at 20 weeks of pregnancy comes to the antenatal OPD with burning micturition. Urine culture grows more than 10^5 CFU/mL of E. coli, sensitive to every agent tested. Which oral antibiotic is the most appropriate?

A 60-year-old man in the ICU has a central line infection. Blood culture grows Enterococcus faecium resistant to both vancomycin and teicoplanin. Which change explains this resistance?

A 24-year-old woman at 16 weeks of pregnancy has a VDRL titre of 1:32 and a positive TPHA in the antenatal clinic. She had anaphylaxis after a penicillin injection three years ago. What is the best next step in treatment?

A 68-year-old man with COPD on sustained-release theophylline is given ciprofloxacin for a urinary infection. On day 3 he has palpitations, vomiting and a generalised seizure. His serum theophylline level is 32 µg/mL (target 10-20 µg/mL). What explains the raised level?

A 3-year-old girl with acute otitis media still has fever and a bulging red eardrum after 3 days of high-dose amoxicillin. Fluid from tympanocentesis grows non-typeable Haemophilus influenzae that produces beta-lactamase. Which antibiotic is the most appropriate change?

A 64-year-old diabetic woman with an indwelling catheter has urosepsis with BP 88/56 mmHg. Blood culture grows Klebsiella pneumoniae that produces an extended-spectrum beta-lactamase; it is resistant to ceftriaxone and sensitive to meropenem. Which drug is the most reliable choice?

A 74-year-old man on long-term prednisolone for COPD is given levofloxacin for a chest infection. On day 5 he feels a sudden snap at the back of his left heel while climbing stairs and cannot stand on tiptoe on that side. Calf squeeze does not move the foot. What has most likely happened?

A 6-day-old term neonate is admitted with suspected sepsis. Serum bilirubin is 16 mg/dL. The resident writes for IV ceftriaxone, but the consultant changes it to cefotaxime. What is the reason for the change?

A 22-year-old student in Delhi with 8 days of fever has Salmonella Typhi in a blood culture. The isolate is resistant to nalidixic acid and has a raised ciprofloxacin MIC, and his fever settled only slowly on ciprofloxacin. What is the most likely mechanism of this reduced susceptibility?

A 72-year-old woman given oral clindamycin after drainage of a dental abscess develops 8 watery stools a day, fever and a TLC of 14,000/µL on day 6. Stool is positive for Clostridioides difficile toxin. Clindamycin is stopped. Which treatment is the most appropriate?

A 30-year-old man with HIV is treated for Pneumocystis pneumonia with high-dose IV cotrimoxazole. On day 5 his serum potassium is 6.4 mmol/L; serum creatinine and blood glucose are normal and he takes no other drugs. How does the drug raise potassium?

A 40-year-old man with MRSA bacteraemia is given IV vancomycin 1 g run in over 15 minutes. Near the end of the infusion he develops flushing and an itchy red rash over his face, neck and upper chest, with mild hypotension. There is no wheeze or stridor. How should the next dose be given?

A 45-year-old woman on sertraline for depression is started on IV linezolid for a complicated MRSA skin infection. On day 2 she is agitated and tremulous, with sweating, temperature 38.8°C, pulse 124/min, hyperreflexia and inducible ankle clonus. How did linezolid contribute to this reaction?

A 5-year-old boy in Pune has pneumococcal meningitis. The CSF isolate of Streptococcus pneumoniae has a penicillin MIC of 2 µg/mL and is beta-lactamase negative. What is the mechanism of its penicillin resistance?