Station 8 — Rabbit Eye Pharmacology: Identify the Unknown Drug

Rapid Revision • Practical Pharmacology • Interpretation • Viva

🎯 Task

CANDIDATE TASK

Examine the test and control rabbit eyes provided. Compare the pupil, conjunctiva, corneal reflex and light reflex, interpret the observed pattern and identify the most likely drug applied to the test eye.

⚡ Model Station Answer

What to say in the exam

Comparison: I would compare the test eye directly with the control eye for pupil size and conjunctival appearance.

Reflexes: I would assess the corneal reflex and light reflex in both eyes.

Interpretation: I would combine these findings rather than identify the drug from pupil size alone.

Conclusion: I would then state the most likely autonomic or local anaesthetic drug producing the observed pattern.

🔎 Stepwise Procedure / Approach

Identify which eye is the test eye and which serves as the control.
Observe both eyes from the same distance and compare pupil size: dilated, constricted or unchanged.
Inspect the conjunctiva for congestion, blanching or no obvious change.
Assess the light reflex by observing pupillary constriction in response to light.
Assess the corneal reflex carefully using the method demonstrated at the station, avoiding corneal injury.
Combine the findings into a pattern: pupil change + conjunctival change + light reflex + corneal reflex.
Match the pattern with the expected pharmacological action and state the most likely unknown drug.

Exam point: Do not identify the drug from mydriasis or miosis alone. Reflex findings are especially useful for distinguishing drugs producing similar pupil changes.

🖼️ Visual Learning

VISUAL 1

Rabbit Eye Examination and Drug-Pattern Recognition

🎥 Practical Video

🎥 Practical Demonstration — Effect of Atropine, Pilocarpine and Local Anaesthetic on Rabbit Eye

Source: Peshawar Medic • MBBS Pharmacology Practical

This practical demonstration shows drug application to the rabbit eye and the observable ocular responses useful for recognizing common unknown drugs during a pharmacology practical station.

▶ Watch Practical Video

📝 Important Viva Questions

Q1. Why must the test eye be compared with the control eye?

Answer: The control eye provides the animal’s baseline pupil size, conjunctival appearance and reflex responses, allowing drug-induced changes to be identified reliably.

Q2. What ocular effect does atropine produce?

Answer: Atropine produces mydriasis by blocking muscarinic receptors in the sphincter pupillae and markedly impairs or abolishes the pupillary light response.

Q3. What characteristic pupil response is produced by pilocarpine?

Answer: Pilocarpine causes miosis by stimulating muscarinic receptors and contracting the sphincter pupillae.

Q4. How can adrenaline be distinguished from atropine when both produce mydriasis?

Answer: Adrenaline may produce conjunctival blanching from α-mediated vasoconstriction, while the parasympathetic sphincter mechanism remains functional; atropine directly blocks the muscarinic light-response pathway.

Q5. Which finding suggests that the unknown drug is a local anaesthetic?

Answer: Loss of the corneal reflex with little or no primary effect on pupil size strongly suggests local corneal anaesthesia.

Q6. Why is pupil size alone insufficient to identify the unknown drug?

Answer: Different drugs can produce similar mydriasis or miosis. The light reflex, corneal reflex and conjunctival changes help distinguish their mechanisms.

Q7. What is the safest examination principle when testing the corneal reflex?

Answer: Use the gentle technique demonstrated for the practical and avoid forceful or traumatic contact with the cornea. The purpose is to assess corneal sensation without causing injury.