Topical Pilocarpine 1% administration failed to produce papillary constriction in a patient who had large dilated pupil. What could be most probable reason
## Core Concept
The question tests the understanding of **pupil constriction mechanisms** and the effects of **pilocarpine**, a muscarinic receptor agonist. Pilocarpine works by stimulating muscarinic receptors in the eye, leading to contraction of the **sphincter pupillae muscle** and subsequent pupil constriction.
## Why the Correct Answer is Right
The correct answer, **D. Denervation of the iris sphincter muscle**, implies a scenario where the iris sphincter muscle is unable to respond to pilocarpine due to a lack of innervation. Normally, pilocarpine acts on muscarinic receptors in the iris sphincter muscle to cause pupil constriction. However, if the iris sphincter muscle is denervated, it cannot respond to pilocarpine, leading to a failure of pupil constriction.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because a normal pupil would constrict in response to pilocarpine. The question specifically mentions a failure to produce papillary constriction, suggesting an abnormality.
- **Option B:** This option refers to a condition where the iris is damaged or absent, but it does not directly relate to the denervation of the iris sphincter muscle. While it could affect pupil reaction, it's not the most direct explanation for a lack of response to pilocarpine.
- **Option C:** This option might seem plausible because a paralyzed ciliary muscle would affect accommodation. However, pilocarpine's effect on pupil constriction is through the iris sphincter muscle, not the ciliary muscle.
## Clinical Pearl / High-Yield Fact
A key point to remember is that **pupil constriction** can be affected by various factors including nerve supply to the iris sphincter muscle. Denervation of this muscle can lead to a fixed, dilated pupil that does not respond to light or miotic agents like pilocarpine. This can be seen in conditions like **Adie's tonic pupil** or after certain types of eye surgery or trauma.
## Correct Answer: D. Denervation of the iris sphincter muscle