Sulfonamides – mechanism, resistance, and uses
Sulfonamides are PABA analogues that inhibit bacterial dihydropteroate synthase, blocking folate synthesis and impairing DNA production. In B.Pharm studies, focus on their mechanism (competitive inhibition of folate pathway), synergy with trimethoprim (sequential blockade), resistance mechanisms (mutated DHPS, increased PABA, plasmid-encoded sul genes, efflux), pharmacokinetics (absorption, acetylation, renal excretion), clinical uses (UTIs, nocardiosis, Pneumocystis/taxoplasma prophylaxis), adverse effects (hypersensitivity, hemolysis in G6PD deficiency, crystalluria, kernicterus), and monitoring/drug interactions. Understanding these concepts is essential for rational use and managing resistance. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which enzyme is directly inhibited by sulfonamides in bacteria?
- Dihydropteroate synthase
- Dihydrofolate reductase
- DNA gyrase
- RNA polymerase
Correct Answer: Dihydropteroate synthase
Q2. Sulfonamides are structural analogues of which substrate?
- Para-aminobenzoic acid (PABA)
- Folic acid
- Thymidine
- Para-hydroxybenzoic acid
Correct Answer: Para-aminobenzoic acid (PABA)
Q3. The combination of sulfamethoxazole with trimethoprim is synergistic because:
- They inhibit consecutive steps in folate synthesis
- Both inhibit dihydropteroate synthase
- They both increase PABA levels
- They prevent drug efflux
Correct Answer: They inhibit consecutive steps in folate synthesis
Q4. A common mechanism of bacterial resistance to sulfonamides is:
- Mutation in dihydropteroate synthase reducing drug binding
- Overexpression of dihydrofolate reductase
- Inactivation by beta-lactamase
- Alteration of 30S ribosomal subunit
Correct Answer: Mutation in dihydropteroate synthase reducing drug binding
Q5. Which plasmid-encoded genes are commonly associated with sulfonamide resistance?
- sul1 and sul2
- blaTEM and blaSHV
- mecA and vanA
- tetA and tetB
Correct Answer: sul1 and sul2
Q6. Sulfonamides are generally classified as bacteriostatic because they:
- Inhibit bacterial growth by blocking folate synthesis
- Cause rapid lysis of bacterial cell wall
- Directly cleave bacterial DNA
- Disrupt bacterial cell membrane integrity
Correct Answer: Inhibit bacterial growth by blocking folate synthesis
Q7. Which sulfonamide is commonly used in combination with trimethoprim (co-trimoxazole)?
- Sulfamethoxazole
- Sulfadiazine
- Sulfisoxazole
- Sulfadoxine
Correct Answer: Sulfamethoxazole
Q8. Which clinical indication is co-trimoxazole commonly used for?
- Urinary tract infections
- Invasive candidiasis
- Viral pharyngitis
- Uncomplicated influenza
Correct Answer: Urinary tract infections
Q9. Sulfonamides may precipitate in renal tubules causing crystalluria; which measure reduces this risk?
- Maintain good hydration and urine alkalinization
- Administer with acidic beverages
- Give as a single large dose
- Avoid concomitant antibiotics
Correct Answer: Maintain good hydration and urine alkalinization
Q10. Which adverse effect is strongly associated with sulfonamides and requires immediate drug discontinuation?
- Stevens-Johnson syndrome
- Transient nausea
- Mild headache
- Rhinorrhea
Correct Answer: Stevens-Johnson syndrome
Q11. Sulfonamides are contraindicated in late pregnancy and neonates primarily because of:
- Risk of kernicterus due to displacement of bilirubin
- Risk of neural tube defects
- Induction of labor
- Impaired fetal bone growth
Correct Answer: Risk of kernicterus due to displacement of bilirubin
Q12. Which patient characteristic increases risk of hemolysis with sulfonamides?
- G6PD deficiency
- Hypothyroidism
- Hyperlipidemia
- Renal artery stenosis
Correct Answer: G6PD deficiency
Q13. Sulfonamides are metabolized primarily by which process in the liver?
- N-acetylation
- Glucuronidation
- Hydroxylation by CYP450
- Reduction
Correct Answer: N-acetylation
Q14. A long-acting sulfonamide commonly used for malaria prophylaxis historically is:
- Sulfadoxine
- Sulfamethoxazole
- Sulfisoxazole
- Sulfadiazine
Correct Answer: Sulfadoxine
Q15. Which laboratory test should be monitored during prolonged sulfonamide therapy?
- Complete blood count for agranulocytosis
- Serum amylase for pancreatitis
- Serum potassium for hyperkalemia
- Thyroid function tests
Correct Answer: Complete blood count for agranulocytosis
Q16. In sulfonamide resistance due to increased PABA production, the impact is that:
- Excess PABA outcompetes the drug at the enzyme active site
- PABA is degraded preventing drug uptake
- PABA enhances drug activation inside bacteria
- PABA increases membrane permeability
Correct Answer: Excess PABA outcompetes the drug at the enzyme active site
Q17. Sulfadiazine is particularly useful in combination therapy for which infection?
- Toxoplasma gondii encephalitis (with pyrimethamine)
- Candida albicans bloodstream infection
- Influenza A pneumonia
- Herpes zoster
Correct Answer: Toxoplasma gondii encephalitis (with pyrimethamine)
Q18. Which statement about the spectrum of sulfonamides is true?
- They have broad activity against many Gram-positive and Gram-negative bacteria but with variable resistance
- They are highly active only against obligate anaerobes
- They are the treatment of choice for Pseudomonas aeruginosa infections
- They are antiviral agents
Correct Answer: They have broad activity against many Gram-positive and Gram-negative bacteria but with variable resistance
Q19. A major drug interaction of sulfonamides is potentiation of which anticoagulant?
- Warfarin
- Heparin
- Low molecular weight heparin
- Direct thrombin inhibitors
Correct Answer: Warfarin
Q20. Which pharmacokinetic property explains once-daily dosing of sulfadoxine in some regimens?
- Long elimination half-life
- Poor oral absorption
- High hepatic extraction ratio
- Rapid renal clearance
Correct Answer: Long elimination half-life
Q21. Dapsone is related to sulfonamides but differs because it primarily:
- Is a sulfone that inhibits dihydropteroate synthase and has anti-inflammatory effects
- Is a beta-lactam antibiotic
- Acts as a macrolide protein synthesis inhibitor
- Is an antifungal agent
Correct Answer: Is a sulfone that inhibits dihydropteroate synthase and has anti-inflammatory effects
Q22. Which adverse hematologic effect is associated with trimethoprim-sulfamethoxazole due to folate antagonism?
- Megaloblastic anemia
- Polycythemia
- Leukocytosis
- Thrombocytosis
Correct Answer: Megaloblastic anemia
Q23. The primary reason sulfonamides are ineffective against human cells is:
- Humans obtain folate from the diet and do not synthesize it de novo
- Human cells lack dihydropteroate synthase altogether
- Sulfonamides cannot cross human cell membranes
- Human cells rapidly degrade sulfonamides
Correct Answer: Humans obtain folate from the diet and do not synthesize it de novo
Q24. Which sulfonamide is preferred for rapid urinary antisepsis due to high solubility and short half-life?
- Sulfisoxazole
- Sulfadoxine
- Sulfadiazine
- Sulfamethoxazole
Correct Answer: Sulfisoxazole
Q25. Resistance caused by plasmid-borne sul genes typically leads to:
- Production of altered dihydropteroate synthase with reduced drug affinity
- Increased penetration of sulfonamides
- Enzymatic degradation of sulfonamides
- Overproduction of DNA gyrase
Correct Answer: Production of altered dihydropteroate synthase with reduced drug affinity
Q26. Which monitoring is important when using high-dose sulfadiazine therapy?
- Renal function and urine pH
- Fasting blood glucose
- Thyroid stimulating hormone
- Cardiac enzymes
Correct Answer: Renal function and urine pH
Q27. Trimethoprim blocks which enzyme, complementing sulfonamide action?
- Dihydrofolate reductase
- Thymidylate synthase
- Dihydropteroate synthase
- RNA polymerase
Correct Answer: Dihydrofolate reductase
Q28. A serious but rare blood dyscrasia reported with sulfonamides is:
- Agranulocytosis
- Polycythemia vera
- Essential thrombocythemia
- Hemochromatosis
Correct Answer: Agranulocytosis
Q29. Which population should avoid sulfonamides due to high risk of neonatal toxicity?
- Newborn infants in the first month of life
- Adolescents aged 13–18 years
- Adults over 65 only
- Pregnant women in the first trimester only
Correct Answer: Newborn infants in the first month of life
Q30. Which approach helps limit development of sulfonamide resistance in clinical practice?
- Use narrow-spectrum agents guided by culture and sensitivity when possible
- Use high-dose sulfonamides for all infections empirically
- Combine sulfonamides with topical steroids routinely
- Avoid all antibacterial stewardship measures
Correct Answer: Use narrow-spectrum agents guided by culture and sensitivity when possible
