## **Core Concept**
Schirmer's test is used to assess tear production in the eyes, specifically to evaluate the function of the lacrimal gland. The test involves placing a strip of filter paper under the eyelid to measure tear production. The underlying principle is related to the innervation of the lacrimal gland.

## **Why the Correct Answer is Right**
The correct answer, **parasympathetic nervous system (or specifically, the greater petrosal nerve, a branch of the facial nerve)**, is right because the lacrimal gland receives parasympathetic innervation from the greater petrosal nerve. This nerve is a branch of the facial nerve (cranial nerve VII) and plays a crucial role in stimulating tear production. The parasympathetic stimulation leads to the release of acetylcholine, which acts on muscarinic receptors in the lacrimal gland to produce tears.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **sympathetic nervous system** primarily controls the blood vessels and has a minor role in tear production, mainly related to the regulation of blood flow rather than the secretion of tears. Thus, it is not the primary nerve responsible for tear production abnormalities detected by Schirmer's test.
- **Option B:** The **trigeminal nerve (cranial nerve V)** is involved in sensory innervation of the face, including the eye, but it is not directly responsible for the parasympathetic stimulation of the lacrimal gland.
- **Option D:** The **optic nerve (cranial nerve II)** is responsible for transmitting visual information and does not have a role in tear production.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Schirmer's test can help diagnose conditions like Sjögren's syndrome, which affects tear and saliva production. The test is specifically looking at the function of the **greater petrosal nerve**, a branch of the facial nerve, which is crucial for parasympathetic stimulation of the lacrimal gland.

## **Correct Answer:** C. parasympathetic nervous system (or the facial nerve through its branch, the greater petrosal nerve).