**Core Concept**
Peripheral iridectomy is a surgical procedure where a small part of the iris is removed to facilitate the drainage of aqueous humor. This procedure is crucial in certain conditions where the iris is obstructing the drainage of aqueous humor, leading to increased intraocular pressure.

**Why the Correct Answer is Right**
Peripheral iridectomy is a must in cases of **congenital glaucoma**. In congenital glaucoma, the trabecular meshwork is often blocked by the iris, leading to increased intraocular pressure. By performing a peripheral iridectomy, the surgeon can remove the obstructing iris tissue and improve aqueous humor drainage, thereby reducing intraocular pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because peripheral iridectomy is not typically performed in cases of primary open-angle glaucoma, where the iris is not obstructing the drainage of aqueous humor.
* **Option B:** This option is incorrect because peripheral iridectomy is not a standard treatment for angle-closure glaucoma, where the iris is blocking the trabecular meshwork. Instead, laser iridotomy is often performed to create a hole in the iris and improve aqueous humor drainage.
* **Option C:** This option is incorrect because peripheral iridectomy is not a common treatment for secondary glaucoma, where the intraocular pressure is increased due to other underlying conditions such as uveitis, trauma, or tumors.

**Clinical Pearl / High-Yield Fact**
Peripheral iridectomy is a critical procedure in the management of congenital glaucoma, and it should be performed before any other glaucoma surgery to ensure proper drainage of aqueous humor.

**Correct Answer:** C.