MCQ on Structure-Activity Relationships (SAR) of Drugs

🟢 EASY (1–10)

1. What does SAR stand for in medicinal chemistry?
   a) Simple Activity Report
   b) Structure Activity Relationship ✅
   c) Structural Atomic Ratio
   d) Systemic Analysis Rule
   Explanation: SAR is the study of how the structure of a molecule affects its biological activity.
2. Which functional group is essential for the analgesic activity of paracetamol?
   a) Carboxylic acid
   b) Hydroxyl group
   c) Amide group ✅
   d) Nitrate group
   Explanation: The amide group in paracetamol is crucial for its analgesic effect.
3. The biological activity of a drug is primarily dependent on:
   a) Color
   b) Odor
   c) Structure ✅
   d) Solubility
   Explanation: Small changes in structure can significantly impact activity.
4. Replacement of a hydrogen atom by a methyl group in a drug can affect:
   a) pH only
   b) Molecular weight only
   c) Lipophilicity and activity ✅
   d) Only boiling point
   Explanation: Methylation can increase lipophilicity and sometimes activity.
5. In SAR, the part of the molecule responsible for its biological activity is called:
   a) Receptor
   b) Carrier
   c) Pharmacophore ✅
   d) Chromophore
   Explanation: The pharmacophore is the essential structural feature required for drug action.
6. Which of the following is a typical modification in SAR studies?
   a) Changing color
   b) Substituting side chains ✅
   c) Freezing the molecule
   d) Increasing container size
   Explanation: Side chain modifications are common in SAR to optimize drug activity.
7. Bioisosteres are used in SAR to:
   a) Decrease solubility
   b) Reduce activity
   c) Mimic original functionality ✅
   d) Improve taste
   Explanation: Bioisosteres are chemical groups with similar properties used to modify activity or reduce toxicity.
8. Which factor is NOT commonly modified in SAR?
   a) Hydrophobicity
   b) Electronic properties
   c) Size
   d) Smell ✅
   Explanation: Smell is not relevant to SAR.
9. Increasing the lipophilicity of a drug generally enhances its:
   a) Solubility in water
   b) Ability to cross membranes ✅
   c) Toxicity only
   d) Weight only
   Explanation: Lipophilic drugs often cross lipid membranes more easily.
10. Which class of drugs has well-studied SAR models?
    a) Vitamins
    b) Steroids ✅
    c) Sugars
    d) Amino acids
    Explanation: Steroids like corticosteroids and sex hormones have well-characterized SAR.

🟡 MODERATE (11–20)

11. What is the primary goal of SAR analysis?
    a) Increase drug price
    b) Reduce color change
    c) Optimize drug potency and selectivity ✅
    d) Increase tablet size
    Explanation: SAR helps develop more potent and specific drugs.
12. Which modification usually enhances CNS activity?
    a) Adding carboxylic acid
    b) Reducing lipophilicity
    c) Increasing molecular weight
    d) Increasing lipophilicity ✅
    Explanation: More lipophilic drugs penetrate the blood-brain barrier more effectively.
13. Which functional group is commonly required for anti-inflammatory activity in NSAIDs?
    a) Hydroxyl
    b) Carboxylic acid ✅
    c) Ether
    d) Amine
    Explanation: The carboxyl group is key for interaction with COX enzymes.
14. Bioisosterism is used in SAR to:
    a) Increase odor
    b) Retain activity with reduced toxicity ✅
    c) Remove activity
    d) Increase molecular weight
    Explanation: Replacing groups with bioisosteres can improve safety profiles.
15. Which of the following changes may decrease drug activity in SAR studies?
    a) Correct spatial orientation
    b) Inserting a bulky group near the active site ✅
    c) Maintaining hydrophilic balance
    d) Preserving pharmacophore
    Explanation: Bulky groups can hinder receptor binding.
16. What is a classical bioisostere for –OH group?
    a) –COOH
    b) –NH₂ ✅
    c) –CH₃
    d) –Cl
    Explanation: –NH₂ can mimic hydrogen bonding like –OH.
17. Which of the following properties is crucial in receptor binding?
    a) Molecular smell
    b) Color
    c) Electronic and steric properties ✅
    d) Radioactivity
    Explanation: The fit into the receptor depends on electronic and spatial characteristics.
18. A drug’s ability to bind to its target receptor is known as:
    a) Affinity ✅
    b) Efficacy
    c) Toxicity
    d) Solubility
    Explanation: Affinity measures how well a drug binds to its receptor.
19. Which property helps a drug selectively interact with only one type of receptor?
    a) Weight
    b) Color
    c) Shape and stereochemistry ✅
    d) Melting point
    Explanation: Proper shape ensures selectivity toward specific receptors.
20. In sulfonamides, which group is essential for antibacterial activity?
    a) Hydroxyl
    b) Amino group on the benzene ring ✅
    c) Carboxyl
    d) Ether
    Explanation: The –NH₂ group mimics PABA and is vital for activity.

🔴 HARD (21–30)

21. Which of the following is a non-classical bioisostere?
    a) –OH and –NH₂
    b) Benzene and thiophene ✅
    c) –CH₃ and –C₂H₅
    d) H and F
    Explanation: Thiophene is structurally different but mimics benzene’s properties.
22. A compound binds strongly but shows no effect. It is likely a:
    a) Full agonist
    b) Partial agonist
    c) Antagonist ✅
    d) Enzyme
    Explanation: Antagonists block the receptor without activating it.
23. Steric hindrance in a molecule may:
    a) Increase receptor binding
    b) Have no effect
    c) Decrease receptor interaction ✅
    d) Always increase activity
    Explanation: Bulk near binding sites often reduces affinity.
24. Which stereoisomer of a drug can sometimes be inactive?
    a) Both always active
    b) Enantiomer ✅
    c) Conformer
    d) Isobar
    Explanation: One enantiomer may not fit the receptor and be inactive.
25. SAR studies can help convert:
    a) Inactive compound to water
    b) Non-drugs to drugs ✅
    c) Proteins to amino acids
    d) Drugs into metals
    Explanation: SAR helps identify active structures in lead compounds.
26. Electronic withdrawing groups on aromatic rings in drugs usually:
    a) Decrease acidity
    b) Increase basicity
    c) Increase acidity ✅
    d) Neutralize the ring
    Explanation: Electron-withdrawing groups increase acidity of adjacent protons.
27. What does Quantitative SAR (QSAR) provide?
    a) Graph of color vs structure
    b) Mathematical model linking structure and activity ✅
    c) Physical data only
    d) Solubility prediction only
    Explanation: QSAR quantifies how structural changes influence activity.
28. Which of the following is a critical steric factor in SAR?
    a) Van der Waals volume ✅
    b) Optical density
    c) Boiling point
    d) UV absorption
    Explanation: Volume influences how a molecule fits in a receptor site.
29. Which approach helps optimize both pharmacokinetics and pharmacodynamics?
    a) SAR only
    b) QSAR only
    c) Integrated SAR & ADME modeling ✅
    d) Physical chemistry
    Explanation: Combining SAR with ADME ensures drug-like behavior.
30. Modification of which region in a molecule is least likely to affect activity?
    a) Pharmacophore
    b) Side chains far from binding site ✅
    c) H-bond donor
    d) Charged group
    Explanation: Remote groups are less involved in receptor interaction.