**Core Concept**
The Ex-Press glaucoma shunt is a type of glaucoma drainage device used to reduce intraocular pressure (IOP) in patients with glaucoma. It works by allowing aqueous humor to drain from the anterior chamber into a reservoir, thereby reducing IOP. The Ex-Press shunt is a relatively newer device compared to other glaucoma drainage devices like the Ahmed or Baerveldt shunts.

**Why the Correct Answer is Right**
The Ex-Press glaucoma shunt is composed of a flexible, silicone tube that is inserted into the anterior chamber of the eye. The tube has a small, circular plate at its distal end, which is designed to sit on the sclera. Aqueous humor can then flow from the anterior chamber into the tube and then into a reservoir, which is implanted under the conjunctiva. This allows for reduced IOP and can help to prevent further damage to the optic nerve. The Ex-Press shunt is particularly useful in patients with glaucoma who have failed other treatments.

**Why Each Wrong Option is Incorrect**
**Option A:** The Ex-Press glaucoma shunt is not made up of a single, large plate that sits on the sclera.
**Option B:** The Ex-Press glaucoma shunt does not use a valved system to control the flow of aqueous humor.
**Option C:** The Ex-Press glaucoma shunt is not composed of a rigid, metal tube.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the Ex-Press glaucoma shunt is often used in patients with refractory glaucoma who have failed other treatments, such as trabeculectomy or laser trabeculoplasty. The device is relatively easy to implant and can be very effective in reducing IOP.

**Correct Answer:** C.