**Core Concept**
Glaucoma is a group of eye conditions that damage the optic nerve, often due to abnormally high pressure in the eye (intraocular pressure, IOP). The condition is characterized by progressive optic nerve damage, which can lead to vision loss if left untreated. The patient's presentation with evening halos and headache suggests a possible glaucomatous condition affecting the optic nerve.

**Why the Correct Answer is Right**
The patient's presentation of evening halos, occasional headache, normal IOP, and shallow anterior chamber (AC) is characteristic of **normal-tension glaucoma (NTG)**. In NTG, optic nerve damage occurs despite normal IOP levels. The shallow AC may indicate a relative pupillary block or other mechanisms contributing to NTG. The patient's symptoms and examination findings suggest that the optic nerve is susceptible to damage due to factors other than elevated IOP, such as vascular factors, genetic predisposition, or other underlying conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the patient's presentation. The patient's normal IOP levels rule out primary open-angle glaucoma (POAG), which is typically characterized by elevated IOP.

**Option B:** This option is incorrect because it suggests that the patient's symptoms are due to a secondary glaucoma, which is not supported by the patient's normal IOP levels and shallow AC.

**Option C:** This option is incorrect because it implies that the patient has a narrow angle glaucoma, which is typically characterized by elevated IOP and a crowded AC, not a shallow AC.

**Clinical Pearl / High-Yield Fact**
In normal-tension glaucoma, the optic nerve may be susceptible to damage due to factors other than elevated IOP, such as vascular factors, genetic predisposition, or other underlying conditions. Clinicians should consider these factors when managing patients with NTG.

**Correct Answer: C. Narrow angle glaucoma.**