**Question:** All the following are true about upper eyelid EXCEPT:

A. The upper eyelid is involved in the lacrimal gland reflex, which helps to close the eyelids during tearing
B. The upper eyelid has a prominent medial and lateral canthal tendon insertion
C. The upper eyelid has a thicker skin layer compared to the lower eyelid
D. The upper eyelid has a single layer of skin and mucous membrane, unlike the lower eyelid with two layers

**Correct Answer:** Option C: "The upper eyelid has a thicker skin layer compared to the lower eyelid."

**Core Concept:** The upper eyelid and lower eyelid are anatomically distinct structures with unique features, playing essential roles in maintaining ocular surface health and protecting the eye.

**Why the Correct Answer is Right:** The upper eyelid is densely innervated and plays a crucial role in the lacrimal gland reflex, which is triggered by tearing stimulation and helps to close the eyelids. This reflex ensures the protection of the ocular surface during tearing and prevents excessive tearing.

**Why Each Wrong Option is Incorrect:**

A. The involvement of the upper eyelid in the lacrimal gland reflex is a key feature, making this option incorrect.
B. The medial and lateral canthal tendons ensure the attachment of the eyelids to the orbital rim and are essential for eyelid position and function, not skin thickness.
C. The upper eyelid does not have a thicker skin layer compared to the lower eyelid. In fact, the lower eyelid has a thicker skin layer with two layers: the orbicularis oculi muscle layer and the dermal layer. The dermal layer is thicker in the lower eyelid, providing a thicker skin layer.
D. The dermal layer is thicker in the lower eyelid, making this option incorrect.

**Clinical Pearl:** A thorough understanding of eyelid anatomy and their functions is essential for proper eyelid examination and diagnosis of various ocular and orbital pathologies.