**Core Concept**
In refractory glaucoma, the goal is to reduce intraocular pressure (IOP) when other treatments have failed. Alpha agonists are a class of medications that can decrease IOP by increasing uveoscleral outflow and reducing aqueous humor production.

**Why the Correct Answer is Right**
Alpha agonists, such as brimonidine, work by stimulating alpha-2 adrenergic receptors in the eye. This leads to a reduction in the production of aqueous humor, which in turn decreases IOP. Additionally, alpha agonists increase the outflow of aqueous humor through the uveoscleral pathway, further contributing to their IOP-lowering effect. This mechanism is particularly useful in refractory glaucoma, where other treatments have failed to control IOP.

**Why Each Wrong Option is Incorrect**
**Option A:** Systemic glucocorticoids can actually increase IOP by causing the lens to become more rigid and the ciliary body to produce more aqueous humor. This makes them contraindicated in glaucoma.

**Option B:** ACE inhibitors are used to treat conditions like hypertension and heart failure, but they do not have a direct effect on IOP. While they may be useful in patients with glaucoma who also have hypertension, they are not a primary treatment for glaucoma.

**Option D:** Beta blockers, such as timolol, are used to reduce IOP by decreasing the production of aqueous humor. However, they are not typically used as a first-line treatment for refractory glaucoma, as alpha agonists are often more effective.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that alpha agonists can cause systemic side effects, such as hypotension and bradycardia, due to their systemic absorption. This highlights the importance of careful patient selection and monitoring when using these medications.

**✓ Correct Answer: C. Alpha agonist**