**Core Concept**
The "corn picker's pupil" refers to a specific type of pupil dilation that can occur due to certain plant poisonings. This phenomenon is characterized by a radial or circumferential dilation of the pupils, often described as resembling a "corn picker's pupil" due to its distinctive shape.

**Why the Correct Answer is Right**
The correct answer is related to **Atropa belladonna**, also known as deadly nightshade. The active alkaloids present in this plant, such as atropine and scopolamine, can cause mydriasis (pupil dilation) by inhibiting the parasympathetic nervous system's action on the iris sphincter muscle. This results in a dilated pupil with a characteristic radial or circumferential shape.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a plant that is associated with the corn picker's pupil phenomenon. While there are other plants that can cause pupil dilation, they do not exhibit this specific characteristic.

**Option B:** This option is incorrect because it is not a plant that is commonly associated with pupil dilation or the corn picker's pupil phenomenon.

**Option C:** This option is incorrect because it is a type of mushroom that can cause pupil dilation, but it is not specifically associated with the corn picker's pupil phenomenon.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that **Atropa belladonna** can cause a range of systemic effects, including anticholinergic toxicity, which can manifest as dry mouth, blurred vision, and confusion, in addition to the characteristic pupil dilation.

**Correct Answer: D. Atropa belladonna**