Iris Muscle Response to Light Intensity

Steps:

• Identify diagram X shows dilated pupil, indicating dim light adaptation.
• Recall circular muscles constrict pupil in bright light by contracting; relax in dim light.
• Recall radial muscles dilate pupil in dim light by contracting; relax in bright light.
• Match X to dim light: circular relaxed, radial contracted.

Why C is correct:

• In dim light, radial muscles contract to widen pupil for more light entry, while circular muscles relax, per phototropism response.

Why the others are wrong:

• A: In bright light, radial muscles relax, not contract.
• B: Describes bright light constriction, not dim light dilation.
• D: In dim light, circular muscles relax, not contract.

Final answer: C