Action of drugs explained by the principle of competitive enzyme inhibition is demonstrated by all of the fol… - UPUMS Nsg officer-2023

UPUMS Nsg officer-2023

Pharmacology

Easy

Appeared in: UPUMS Nsg officer-2023

Explanation

British Anti-Lewisite (BAL), also known as dimercaprol, does not act by inhibiting an enzyme competitively.
Its mechanism of action is chelation, where it functions as a chemical antagonist.
BAL contains sulfhydryl (-SH) groups that directly bind to heavy metals like arsenic, mercury, and gold, forming a stable, less toxic complex.
This chelated complex is then excreted by the body, thus BAL reverses the toxic effects of heavy metals on enzymes rather than competing for an enzyme's active site.

Option A: Isoniazid is an example of a competitive inhibitor. It is structurally similar to pyridoxal (Vitamin B6) and competes with it for essential enzymes in the metabolism of mycobacteria, the bacteria that cause tuberculosis.
Option B: Methotrexate is a classic example of a competitive inhibitor used in chemotherapy and for autoimmune diseases. It is a structural analogue of folic acid and inhibits the enzyme dihydrofolate reductase, which is crucial for DNA synthesis and cell replication.
Option C: Sulphonamides are antibacterial drugs that work through competitive inhibition. They are structural analogues of para-aminobenzoic acid (PABA) and inhibit the bacterial enzyme dihydropteroate synthase, preventing bacteria from synthesizing their own folic acid.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Mechanisms of Drug Action: Competitive Enzyme Inhibition vs. Chelation as background academic context rather than a clinical decision trigger.
Understanding a drug's mechanism is vital for safe nursing practice, allowing anticipation of effects, side effects, and potential antidotes.
For competitive inhibitors like Isoniazid, nurses should monitor for signs of peripheral neuropathy and know that pyridoxine (Vitamin B6) is given to mitigate this side effect.
When administering chelating agents like BAL, nurses must monitor vital signs (especially blood pressure), ensure deep intramuscular injection technique, and assess renal function, as the metal-chelate complex is excreted via the kidneys.

How to Approach the Question

First, identify the core concept in the question stem, which is 'competitive enzyme inhibition'. This mechanism involves a drug that structurally resembles a natural substrate and competes for an enzyme's active site.
Next, notice the critical keyword 'EXCEPT'. This signals that you must find the one option that does NOT fit the description of a competitive inhibitor.
Systematically evaluate each option's mechanism of action. Recall or deduce if the drug competes with a substrate.
Analyze Isoniazid (competes with pyridoxal), Methotrexate (competes with folic acid), and Sulphonamides (competes with PABA). All three fit the definition.
Analyze British Anti-Lewisite. Its mechanism is binding directly to heavy metals (chelation), which is a form of chemical antagonism, not competitive enzyme inhibition.
Conclude that British Anti-Lewisite is the exception and therefore the correct answer.

Concept Tested & Keywords

Concept Tested: Mechanisms of Drug Action: Competitive Enzyme Inhibition vs. Chelation
Stem keywords: action of drugs, competitive enzyme inhibition
Lead-in keywords: EXCEPT
Negative lead-in flag: The question asks for the option that is NOT an example of competitive enzyme inhibition.

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