Random screening is an early drug discovery approach where diverse chemical or natural compound libraries are tested in unbiased bioassays to find active “hits”. For B.Pharm students, understanding random screening connects concepts in pharmacology, medicinal chemistry, high-throughput screening, assay design, hit identification, lead generation, toxicity profiling and structure–activity relationships. This method complements target-based and phenotypic strategies, helping identify novel scaffolds, unexpected mechanisms and chemical starting points for optimization. Practical knowledge of screening workflows, library composition, hit validation, false positives, ADME considerations and data analysis is essential for effective lead discovery and risk reduction. This focused set of MCQs with answers will reinforce core principles and exam-ready facts. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary goal of random screening in early drug discovery?
- To validate a pre-defined molecular target using a known inhibitor
- To test diverse compounds in unbiased assays to identify active hits
- To optimize ADME properties of a clinical candidate
- To perform large-scale clinical trials
Correct Answer: To test diverse compounds in unbiased assays to identify active hits
Q2. Which screening approach is most characteristic of “random screening”?
- Target-based high-content imaging
- Phenotypic or unbiased functional assays
- Structure-based virtual screening only
- Clinical endpoint monitoring
Correct Answer: Phenotypic or unbiased functional assays
Q3. In high-throughput screening (HTS), what metric is commonly used to assess assay quality and separation between positive and negative controls?
- pKa value
- Z’-factor
- LogP
- IC50
Correct Answer: Z’-factor
Q4. Which of the following is a major advantage of random (phenotypic) screening over strictly target-based screening?
- Requires detailed target structure information
- More likely to reveal novel mechanisms of action
- Produces fewer off-target effects by design
- Always higher hit rates than target-based assays
Correct Answer: More likely to reveal novel mechanisms of action
Q5. What is a “hit” in the context of random screening?
- A compound confirmed in clinical trials
- A compound that shows desired activity in the primary screening assay
- A compound with perfect ADME/Tox properties
- A computational prediction with no experimental data
Correct Answer: A compound that shows desired activity in the primary screening assay
Q6. Which library property most increases the chance of discovering novel scaffolds in random screening?
- Low chemical diversity
- High structural diversity and novelty
- Only analogs of a single lead
- Only known drugs
Correct Answer: High structural diversity and novelty
Q7. What is a common cause of false positives in fluorescence-based screening assays?
- Proper target engagement
- Compound autofluorescence or quenching
- Correct assay controls
- High Z’-factor
Correct Answer: Compound autofluorescence or quenching
Q8. What is a counter-screen used for after an initial hit from random screening?
- To increase plate throughput
- To test the hit in an orthogonal assay and remove assay artifacts
- To immediately start human trials
- To calculate logP
Correct Answer: To test the hit in an orthogonal assay and remove assay artifacts
Q9. What does the term “PAINS” refer to in screening libraries?
- Promiscuous Aggregating Inhibitors of Nuclear Signaling
- Pan-Assay Interference Compounds that produce misleading assay readouts
- Proteins Affected In Normal Screening
- Preferred Analytical Inert Substances
Correct Answer: Pan-Assay Interference Compounds that produce misleading assay readouts
Q10. Which follow-up step is essential after identifying primary hits in random screening?
- Skip validation and proceed to clinical trials
- Hit confirmation with dose–response and orthogonal assays
- Directly file a patent without further testing
- Outsource to a CRO without documentation
Correct Answer: Hit confirmation with dose–response and orthogonal assays
Q11. Why is DMSO tolerance important in HTS assays?
- DMSO always stabilizes proteins
- Compounds are typically stored in DMSO; tolerance prevents solvent effects on assay readout
- DMSO eliminates the need for controls
- DMSO increases signal-to-noise ratio invariably
Correct Answer: Compounds are typically stored in DMSO; tolerance prevents solvent effects on assay readout
Q12. What does “hit rate” mean in a screening campaign?
- The number of compounds screened per hour
- The percentage of screened compounds that meet activity criteria
- The toxicity percentage in animal studies
- The fraction of compounds that are water-soluble
Correct Answer: The percentage of screened compounds that meet activity criteria
Q13. Which assay parameter describes the magnitude of signal difference between positive and negative controls?
- Assay window or signal window
- pIC50
- Partition coefficient
- LC50
Correct Answer: Assay window or signal window
Q14. What role does miniaturization play in HTS for random screening?
- Increases reagent costs dramatically
- Reduces reagent consumption and increases throughput
- Prevents automation
- Always reduces data quality
Correct Answer: Reduces reagent consumption and increases throughput
Q15. Which computational filter is useful during library design to avoid frequent hitters?
- PAINS filters
- LogP maximization
- Increase molecular weight filter to >1000
- None — filters are not used
Correct Answer: PAINS filters
Q16. What is an “orthogonal assay” in the context of hit validation?
- An assay using the same detection principle as the primary screen
- An independent assay that measures the same biological effect using a different readout
- An assay run in a different laboratory without controls
- An in silico docking simulation only
Correct Answer: An independent assay that measures the same biological effect using a different readout
Q17. Why is early ADME/Tox profiling important in hit-to-lead progression?
- To determine plate layout
- To identify liabilities like metabolic instability and toxicity early, saving time and cost
- Because ADME/Tox is required for primary screening
- To guarantee FDA approval
Correct Answer: To identify liabilities like metabolic instability and toxicity early, saving time and cost
Q18. Which phenomenon often causes false negatives in screening?
- Appropriate assay sensitivity
- Insufficient compound concentration or poor compound solubility
- Using orthogonal assays
- High hit confirmation rates
Correct Answer: Insufficient compound concentration or poor compound solubility
Q19. In random screening, what is an advantage of including natural product extracts in libraries?
- They are always easy to synthesize and optimize
- They often contain unique, biologically evolved scaffolds and complex chemotypes
- They never show assay interference
- They reduce assay complexity
Correct Answer: They often contain unique, biologically evolved scaffolds and complex chemotypes
Q20. What is the significance of signal-to-background ratio in a screening assay?
- It quantifies assay robustness; higher ratio improves detection of true hits
- It measures compound solubility
- It is only relevant for clinical assays
- It determines molecular weight of hits
Correct Answer: It quantifies assay robustness; higher ratio improves detection of true hits
Q21. Why are orthogonal biochemical assays used after phenotypic hits are found?
- To confirm compound mechanism and exclude nonspecific effects
- To immediately file patents
- To increase the library size
- To discard hits without testing
Correct Answer: To confirm compound mechanism and exclude nonspecific effects
Q22. What is an aggregating compound and why is it problematic in screening?
- A compound that forms colloidal aggregates that nonspecifically inhibit proteins, causing false positives
- A perfectly specific inhibitor
- A compound that degrades rapidly in buffer
- A compound with ideal solubility
Correct Answer: A compound that forms colloidal aggregates that nonspecifically inhibit proteins, causing false positives
Q23. Which parameter is commonly determined in hit confirmation experiments to quantify potency?
- pH
- IC50 or EC50
- Melting point
- Retention time
Correct Answer: IC50 or EC50
Q24. How does combinatorial chemistry support random screening efforts?
- By generating diverse libraries of analogs for SAR exploration and hit optimization
- By reducing compound diversity
- By guaranteeing clinical safety
- By eliminating the need for assays
Correct Answer: By generating diverse libraries of analogs for SAR exploration and hit optimization
Q25. Which automation component is most critical for reproducible HTS?
- Manual pipetting
- Robotic liquid handlers and precise plate readers
- Hand labeling of tubes
- Audio control systems
Correct Answer: Robotic liquid handlers and precise plate readers
Q26. What is the purpose of including positive and negative controls on screening plates?
- Only for aesthetics
- To normalize data, monitor assay performance and calculate metrics like Z’-factor
- To increase the number of hits artificially
- To slow down the screening process
Correct Answer: To normalize data, monitor assay performance and calculate metrics like Z’-factor
Q27. Why is data curation important after a random screening run?
- To randomly delete data
- To remove artifacts, flag suspicious results, and prepare reliable hit lists for follow-up
- To bypass hit confirmation
- To immediately publish raw results
Correct Answer: To remove artifacts, flag suspicious results, and prepare reliable hit lists for follow-up
Q28. Which screening scale is most appropriate for an initial broad random screen of a medium-sized academic library?
- Single-compound crystallography
- High-throughput plate-based assay (384- or 1536-well)
- Large Phase III clinical study
- In vivo toxicology in multiple species
Correct Answer: High-throughput plate-based assay (384- or 1536-well)
Q29. How does early structure–activity relationship (SAR) analysis assist hit progression?
- By identifying which structural changes improve potency, selectivity and ADME properties
- By preventing any chemical modification of hits
- By only focusing on biological assays
- By eliminating the need for medicinal chemistry
Correct Answer: By identifying which structural changes improve potency, selectivity and ADME properties
Q30. Which consideration improves the translational value of hits from random screening to in vivo models?
- Ignoring solubility and exposure
- Early assessment of pharmacokinetics, solubility and metabolic stability
- Only measuring in vitro potency without context
- Assuming all hits have good oral bioavailability
Correct Answer: Early assessment of pharmacokinetics, solubility and metabolic stability
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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