Question

Which of the following statement is NOT CORRECT?

• A. Fluoroquinolones absorption is reduced with polyvalent metal ions
• B. Sulfamethaxazole is a competitive inhibitor of enzyme dihydropteroate synthase
• C. 1,3-Diaminoinositol is the pharmacophore moiety in aminoglycosides
• D. Macrocyclic lactone ring is present in tetracycline antibacterials

Hint:

Associate these key structural terms with their respective antibiotic classes: - Tetracyclines: Four fused carbocyclic rings (naphthacene). - Macrolides: Large macrocyclic lactone ring (usually 14 to 16-membered). - Aminoglycosides: Aminocyclitol ring (2-deoxystreptamine 1,3-diaminoinositol).

Answer 1

The Correct Option is D

Concept: This question requires evaluating statements across different classes of antimicrobial agents to identify the single chemically incorrect assertion. Let us review each option systematically:
• Option (A): Fluoroquinolones absorption is reduced with polyvalent metal ions. This statement is completely correct. Fluoroquinolones (such as Ciprofloxacin) contain a 4-oxo-1,4-dihydroquinoline-3-carboxylic acid core. The carbonyl oxygen at position 4 and the carboxylic acid oxygen at position 3 form a stable chelate complex with polyvalent metal cations (\(\text{Al}^{3+}, \text{Mg}^{2+}, \text{Ca}^{2+}, \text{Fe}^{2+}\)) present in antacids, dairy products, or iron supplements. This chelated complex is insoluble and cannot be absorbed by the gastrointestinal tract, lowering the drug's bioavailability.
• Option (B): Sulfamethoxazole is a competitive inhibitor of enzyme dihydropteroate synthase. This statement is completely correct. Sulfamethoxazole is a structural analogue of para-aminobenzoic acid (PABA). It competitively binds to the enzyme dihydropteroate synthase (DHPS), blocking the condensation of PABA with dihydropteridine pyrophosphate. This halts bacterial folic acid synthesis.
• Option (C): 1,3-Diaminoinositol is the pharmacophore moiety in aminoglycosides. This statement is correct. Aminoglycoside antibiotics consist of amino sugars linked glycosidically to a central aminocyclitol ring. In most clinically used aminoglycosides (like Gentamicin, Amikacin, and Tobramycin), this aminocyclitol ring is 2-deoxystreptamine, which is structurally derived from a modified 1,3-diaminoinositol system.
• Option (D): Macrocyclic lactone ring is present in tetracycline antibacterials. This statement is INCORRECT. Tetracyclines (such as Doxycycline) are named for their core structure of four fused hydrocarbon rings (a naphthacene carboxamide system). They do not contain a macrocyclic lactone ring. Instead, a large macrocyclic lactone ring is the defining feature of Macrolide antibiotics (such as Erythromycin, Clarithromycin, and Azithromycin). Conclusion: Because macrocyclic lactone rings are characteristic of macrolides rather than tetracyclines, Option (D) is the incorrect statement and the correct answer.