BPSC 69th CCE Prelims General Studies · Question 28 of 144

Correct — B, Molecular targets. The stem is lifted almost word for word from the NCERT Class 12 Chemistry chapter 'Chemistry in Everyday Life', which sets out four different bases on which drugs may be classified and then names one of them as the most useful for medicinal chemists. The four are these. Classification on the basis of pharmacological effect gathers every drug that produces the same biological result, so all analgesics sit together whatever their chemistry — the arrangement a prescribing doctor wants, because it shows the full range of drugs available for one kind of problem. Classification on the basis of drug action groups drugs by the biochemical process they interfere with, which is how all antihistamines come to be one class. Classification on the basis of chemical structure groups molecules that share a common skeleton, as the sulpha drugs all share the sulphanilamide nucleus. And classification on the basis of molecular targets groups drugs by the biomolecule they actually bind to — proteins in the form of enzymes and receptors, and beyond them nucleic acids, carbohydrates and lipids. NCERT's own conclusion is that drugs possessing some common structural features may share the same mechanism of action on a particular target, and that this target-based classification is the most useful one for medicinal chemists. The reason is occupational. A medicinal chemist is not choosing a therapy; he or she is designing a molecule, and only the target tells you the three-dimensional pocket the molecule has to fit, the charges it has to complement and the bond it has to make or block. Aspirin is understood as a drug that acetylates a serine residue in the active site of cyclo-oxygenase; cimetidine and ranitidine as antagonists that occupy the histamine H2 receptor on the stomach's acid-secreting cells; the sulpha drugs as inhibitors of the bacterial enzyme dihydropteroate synthase. Each of those descriptions is a design brief. 'Analgesic' and 'antacid' are not.

• (a)Pharmacological effect — A perfectly real classification and the first one NCERT lists, which is what makes it the strongest wrong answer here — nothing in it is false. It groups drugs by the effect they produce on the body, so all analgesics form one class and all antipyretics another, and it is the most useful arrangement for a physician because it displays every drug available against a given complaint. Its limitation is precisely what the stem is probing: the class 'analgesic' contains aspirin, morphine and paracetamol, three molecules with different skeletons acting on different targets, so knowing the effect tells a chemist nothing whatever about what to synthesise.
• (c)Chemical structure — Also a genuine basis, and the one a candidate is likeliest to pick after ruling out the pharmacological option, on the reasoning that a chemistry question must have a chemistry answer. It groups drugs sharing a common structural skeleton, which is how the sulphonamides became a named family. But NCERT flags its weakness: drugs built on the same nucleus often, yet not always, share a pharmacological activity, so structure alone does not predict what a molecule will do in the body. Structure is the answer to the question, and the target is the question — which is why the target and not the skeleton organises the discipline.
• (d)None of the above — Wrong because one of the listed bases is exactly the one the textbook singles out. It is worth noticing that a fourth genuine basis, classification by drug action, is missing from the options altogether, which can make a well-prepared candidate suspect the list is incomplete and reach for the escape route. The list being incomplete does not make it wrong: the stem asks which basis is the most useful, and the most useful one is present.

Medicinal chemistry is the branch of chemistry concerned with the design, synthesis and mechanism of substances used as medicines. NCERT defines drugs as chemicals of low molecular mass, roughly 100 to 500 u, which interact with macromolecular targets in the body and produce a biological response; when that response is therapeutic and the substance is safe at the dose used, the drug is called a medicine. The macromolecules they act on are the drug targets, and two kinds dominate. Enzymes are biological catalysts, and a drug that blocks one is an inhibitor: it may compete with the natural substrate for the active site, or bind at an allosteric site and change the shape of the active site so the substrate no longer fits. Receptors are proteins embedded in cell membranes that receive chemical messengers; a drug that mimics the messenger and switches the receptor on is an agonist, and one that occupies the site and blocks the natural messenger is an antagonist. The reason this framework matters beyond the exam is selective toxicity — an antibacterial works only because the target it attacks exists in the bacterium and not in us, which is why the sulpha drugs, aimed at an enzyme of bacterial folate synthesis that human cells do not possess, can be swallowed safely.

The first thing to notice is that this is not a true-or-false question. All three named bases are real classifications used in pharmacology and all three appear in the same textbook paragraph, so eliminating options on correctness will not work — the item is asking which is most useful, and usefulness has to be useful to somebody. That is where the discriminating words sit: 'in medical chemistry', meaning medicinal chemistry, the chemist's discipline rather than the clinician's. Change the profession named in the stem and the answer changes with it. A hospital pharmacist ordering stock or a physician at a bedside is best served by the pharmacological-effect classification, because it lists what is available for a given complaint; a regulator compiling drug-utilisation statistics uses the World Health Organization's Anatomical Therapeutic Chemical system, which deliberately mixes several bases across five hierarchical levels; but a chemist designing a molecule needs to know the shape and chemistry of the binding site, and only the molecular target supplies it. The general exam lesson is worth carrying away: when three options are all factually true, the answer is being decided by a qualifier in the stem, so go back and find it rather than weighing the options against each other. Here the qualifier is four words long and does all the work.

• NCERT Class 12 Chemistry, 'Chemistry in Everyday Life', lists four bases for classifying drugs — pharmacological effect, drug action, chemical structure and molecular targets — and states that classification on the basis of molecular targets is the most useful for medicinal chemists. Note that 'drug action' is not offered among this question's options.
• Drugs are chemicals of low molecular mass, about 100 to 500 u, that interact with macromolecular targets to produce a biological response; they are called medicines only when that response is therapeutic and useful at a non-toxic dose.
• The two dominant classes of drug target are enzymes and receptors, both proteins, with nucleic acids, carbohydrates and lipids making up the remainder. A drug blocking an enzyme is an inhibitor, competitive or allosteric; one acting at a receptor is an agonist if it mimics the natural messenger and an antagonist if it blocks it.
• Worked target examples: aspirin acetylates a serine residue in the active site of cyclo-oxygenase; cimetidine and ranitidine are histamine H2-receptor antagonists that suppress gastric acid secretion; penicillin blocks the transpeptidase that cross-links the bacterial cell wall; sulpha drugs inhibit dihydropteroate synthase, an enzyme of bacterial folate synthesis absent in humans, which is the source of their selective toxicity.
• No single basis is universally best, which is why the stem has to name a profession. The World Health Organization's Anatomical Therapeutic Chemical (ATC) classification, the international standard for drug statistics, deliberately combines organ system with therapeutic, pharmacological and chemical properties across five hierarchical levels.

• Eliminating options for being false when all three are genuinely used classifications — the item is a comparative judgement, not a fact check
• Choosing 'chemical structure' because the stem says chemistry; structure describes the molecule, the target describes the job the molecule has to do
• Missing the qualifier 'in medical chemistry' — for a physician the pharmacological-effect classification really is the most useful one, and the same stem without those words would have a different answer

BPSC took this straight from the NCERT Class 12 chapter and asked it as an almost verbatim recall item, which is characteristic of the science block on the 69th paper — the reward goes to a candidate who has read the textbook line rather than to one who reasons from first principles. Its follow-ups in the same area are usually single-fact drug identifications, as on the 71st CCE in 2025 with the metal compound used against carcinoma. UPSC has never asked the classification itself; it works the same chapter from the applied end, matching named drugs to their uses, as with atropine, ether and nitroglycerine in 1999, or asking which antimicrobial serves in two diseases at once, as with rifampicin in 1995.

• practice — not a real PYQ

  In medicinal chemistry, a drug that binds to a receptor site and blocks the natural chemical messenger from acting is called

  • (a)an agonist
  • (b)an antagonist
  • (c)an allosteric activator
  • (d)a promoter

  Answer(b) an antagonist — it occupies the binding site and prevents the natural messenger from producing its effect, which is how cimetidine and ranitidine cut gastric acid secretion at the histamine H2 receptor. An agonist does the opposite and mimics the messenger; allosteric activators and promoters belong to enzyme kinetics and catalysis, not receptor pharmacology.

• practice — not a real PYQ

  Which of the following is NOT one of the bases on which drugs are classified?

  • (a)Pharmacological effect
  • (b)Molecular targets
  • (c)Chemical structure
  • (d)Cost of manufacture

  Answer(d) Cost of manufacture — the four standard bases are pharmacological effect, drug action, chemical structure and molecular targets. Price is a commercial matter and organises no pharmacological classification, whereas the other three named here are all in regular use.