Para-aminosalicylic acid (PAS) is a classical antitubercular drug with distinct chemistry and defined biological activity, essential for B. Pharm students. PAS (4-amino-2-hydroxybenzoic acid) is a PABA analog featuring aromatic amine, phenolic hydroxyl, and carboxylic acid groups; knowledge of its structure, synthesis, and physicochemical properties aids formulation and analysis. PAS acts as an antimetabolite interfering with folate biosynthesis in Mycobacterium tuberculosis, exhibiting mainly bacteriostatic activity. Important topics include structure-activity relationships (SAR), metabolism (N-acetylation), pharmacokinetics, adverse effects, resistance mechanisms, and analytical methods such as HPLC. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the correct IUPAC name of Para-aminosalicylic acid (PAS)?
- 4-Amino-2-hydroxybenzoic acid
- 2-Amino-4-hydroxybenzoic acid
- 3-Amino-2-hydroxybenzoic acid
- 4-Amino-3-hydroxybenzoic acid
Correct Answer: 4-Amino-2-hydroxybenzoic acid
Q2. Which one is the molecular formula of PAS?
- C7H7NO3
- C7H6NO3
- C8H9NO3
- C6H7NO3
Correct Answer: C7H7NO3
Q3. Which functional groups are present in PAS?
- Carboxylic acid, phenolic hydroxyl, aromatic amine
- Alcohol, ketone, aliphatic amine
- Ether, ester, tertiary amine
- Amide, aldehyde, phenol
Correct Answer: Carboxylic acid, phenolic hydroxyl, aromatic amine
Q4. PAS is chemically classified as a structural analog of which endogenous compound?
- Para-aminobenzoic acid (PABA)
- Folic acid
- Para-hydroxybenzoic acid (PHBA)
- Benzoic acid
Correct Answer: Para-aminobenzoic acid (PABA)
Q5. The primary antimicrobial mechanism of PAS in Mycobacterium tuberculosis involves:
- Interference with folate synthesis as a PABA antimetabolite
- Inhibition of cell wall mycolic acid synthesis
- Disruption of DNA gyrase activity
- Inhibition of protein synthesis at the 30S ribosomal subunit
Correct Answer: Interference with folate synthesis as a PABA antimetabolite
Q6. The antibacterial action of PAS is best described as:
- Bacteriostatic against Mycobacterium tuberculosis
- Bactericidal against gram-positive cocci
- Bactericidal against Mycobacterium tuberculosis
- Bacteriostatic against viruses
Correct Answer: Bacteriostatic against Mycobacterium tuberculosis
Q7. In clinical practice, PAS is typically used as:
- A second-line antitubercular agent for resistant TB
- A first-line single therapy for active TB
- An antifungal for systemic candidiasis
- A prophylactic antibiotic for urinary tract infections
Correct Answer: A second-line antitubercular agent for resistant TB
Q8. Major metabolic pathway of PAS in humans is:
- N-acetylation in the liver
- Sulfation in the kidney
- Glucuronidation in the intestine
- Oxidative deamination in plasma
Correct Answer: N-acetylation in the liver
Q9. The principal route of elimination for PAS and its metabolites is:
- Renal excretion in urine
- Fecal excretion via bile
- Exhalation as CO2
- Sequestration in adipose tissue
Correct Answer: Renal excretion in urine
Q10. The most common adverse effect observed with PAS therapy is:
- Gastrointestinal upset including nausea and diarrhea
- Severe neutropenia as a routine effect
- Permanent ototoxicity
- Acute pancreatitis in most patients
Correct Answer: Gastrointestinal upset including nausea and diarrhea
Q11. Which pharmaceutical form of PAS is used to improve aqueous solubility for oral administration?
- Sodium salt of PAS
- Free acid crystals of PAS
- PAS microparticles suspended in oil
- PAS as a heavy metal complex
Correct Answer: Sodium salt of PAS
Q12. PAS shows cross-relation in mechanism with which antimicrobial class?
- Sulfonamides (both are PABA analogs)
- Fluoroquinolones (DNA gyrase inhibitors)
- Aminoglycosides (30S inhibitors)
- Glycopeptides (cell wall inhibitors)
Correct Answer: Sulfonamides (both are PABA analogs)
Q13. A known mechanism by which Mycobacterium tuberculosis can develop resistance to PAS is:
- Mutations affecting folate pathway enzymes such as DHPS
- Production of β-lactamases
- Overexpression of ribosomal protection proteins
- Alteration of LPS structure
Correct Answer: Mutations affecting folate pathway enzymes such as DHPS
Q14. Which analytical method is routinely used for quantitative estimation of PAS in formulations and plasma?
- High-performance liquid chromatography (HPLC)
- Infrared spectroscopy (IR) only
- Flame photometry
- Polarography exclusively
Correct Answer: High-performance liquid chromatography (HPLC)
Q15. Which structural modification of PAS would most likely decrease its PABA antimetabolite activity?
- Blocking the aromatic amine by acetylation
- Replacing the hydroxyl with hydrogen
- Adding a methyl group to the aromatic ring meta to COOH
- Converting carboxylic acid to its free acid form
Correct Answer: Blocking the aromatic amine by acetylation
Q16. The solubility profile of PAS in water is best described as:
- Moderately soluble; solubility increases as sodium salt
- Highly lipophilic and insoluble in water
- Completely insoluble even as salt forms
- Volatile and gaseous at room temperature
Correct Answer: Moderately soluble; solubility increases as sodium salt
Q17. Which of the following analytical detectors is commonly paired with HPLC for PAS detection in plasma?
- UV detector at appropriate wavelength
- Flame ionization detector (FID)
- Electrochemical detector without derivatization
- Thermal conductivity detector (TCD)
Correct Answer: UV detector at appropriate wavelength
Q18. A commonly reported serious but less frequent adverse effect of PAS is:
- Hepatotoxicity with elevated liver enzymes
- Permanent blindness in all patients
- Irreversible cardiomyopathy immediately on dosing
- Acute renal failure in 100% of cases
Correct Answer: Hepatotoxicity with elevated liver enzymes
Q19. Which synthetic step is typically involved in laboratory preparation of PAS?
- Reduction of a nitro-salicylic intermediate to an amino group
- Direct amination of benzene without prior substitution
- Fischer esterification of aniline
- Grignard addition to a carboxylic acid
Correct Answer: Reduction of a nitro-salicylic intermediate to an amino group
Q20. Which property of PAS is most important when formulating dosage forms to minimize gastric irritation?
- Using enteric-coated or buffered formulations
- Increasing free acid concentration
- Formulating as an oily suspension for IV use
- Making a powdered inhalation product
Correct Answer: Using enteric-coated or buffered formulations
Q21. In structure-activity relationship (SAR) for PAS, the presence of which group is crucial for PABA antimetabolite activity?
- Free aromatic amino group at para position
- Alkyl chain longer than C4
- Quaternary ammonium substituent
- Bulky tert-butyl at ortho position
Correct Answer: Free aromatic amino group at para position
Q22. Which laboratory test can monitor PAS exposure or compliance in a patient sample?
- HPLC assay of urine or plasma for PAS and metabolites
- Gram staining of sputum
- ELISA for anti-PAS antibodies
- Prothrombin time alone
Correct Answer: HPLC assay of urine or plasma for PAS and metabolites
Q23. Which statement about PAS pharmacokinetics is true?
- Orally administered PAS is absorbed and undergoes first-pass hepatic acetylation
- PAS is exclusively administered intravenously due to poor absorption
- PAS is never metabolized and is excreted unchanged only in feces
- PAS accumulates indefinitely in adipose tissue
Correct Answer: Orally administered PAS is absorbed and undergoes first-pass hepatic acetylation
Q24. Which laboratory analytical parameter provides highest specificity for PAS in complex matrices?
- HPLC coupled with UV detection at characteristic wavelength
- Paper chromatography without reference standard
- Simple melting point determination
- Visual color comparison only
Correct Answer: HPLC coupled with UV detection at characteristic wavelength
Q25. PAS therapy is contraindicated or used cautiously in patients with:
- Severe hepatic impairment due to risk of hepatotoxicity
- Myopia requiring immediate ophthalmic surgery
- History of controlled hypertension only
- Routine childhood vaccination scheduled
Correct Answer: Severe hepatic impairment due to risk of hepatotoxicity
Q26. Which excipient strategy can improve PAS palatability and patient compliance in oral formulations?
- Use of sweetening agents and buffering to reduce acidity
- Removing all flavoring to keep raw bitter taste
- Adding high concentrations of alcohol
- Coating tablets with unpalatable bitter coatings
Correct Answer: Use of sweetening agents and buffering to reduce acidity
Q27. Which chromatographic mobile phase modifier is commonly used to improve PAS peak shape in reversed-phase HPLC?
- Use of a low percentage of organic solvent with buffer at appropriate pH
- Pure hexane as eluent
- 100% chloroform without aqueous buffer
- Strong oxidizing agents in mobile phase
Correct Answer: Use of a low percentage of organic solvent with buffer at appropriate pH
Q28. In combination therapy for multidrug-resistant TB, PAS is often combined with which class of drugs?
- Second-line agents such as ethionamide or cycloserine
- Topical antifungals only
- Beta-lactam antibiotics as monotherapy
- Antiviral nucleoside analogs routinely
Correct Answer: Second-line agents such as ethionamide or cycloserine
Q29. Which storage condition is recommended to maintain PAS stability in bulk and formulations?
- Cool, dry place protected from light
- Open to direct sunlight at high humidity
- Storage at high temperature and humidity
- Keep in strong acid to prevent degradation
Correct Answer: Cool, dry place protected from light
Q30. Which structural isomer corresponds to para-aminosalicylic acid?
- 4-Amino-2-hydroxybenzoic acid (amino para to carboxyl, hydroxy at ortho)
- 2-Amino-4-hydroxybenzoic acid (amino ortho to carboxyl)
- 3-Amino-2-hydroxybenzoic acid (meta amino)
- 4-Amino-3-hydroxybenzoic acid (hydroxy meta)
Correct Answer: 4-Amino-2-hydroxybenzoic acid (amino para to carboxyl, hydroxy at ortho)
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.
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