**Core Concept**
Sympathetic ophthalmia is a rare, bilateral granulomatous uveitis that occurs after a penetrating eye injury, typically within weeks to months after the initial trauma. This condition is a result of the immune system's response to the intraocular antigens released during the injury.

**Why the Correct Answer is Right**
The patient's presentation of difficulty in reading with the right eye, three weeks after sustaining a gunshot injury to his left eye, is suggestive of sympathetic ophthalmia. This condition occurs when the immune system mistakenly attacks the healthy eye, leading to inflammation and damage to the uvea, which includes the iris, choroid, and ciliary body. The mechanism involves an autoimmune response, where the immune cells recognize the intraocular antigens as foreign and initiate an inflammatory response. The condition can lead to vision loss in the affected eye, and in severe cases, can also affect the uninjured eye.

**Why Each Wrong Option is Incorrect**
**Option A:** Macular edema is a condition characterized by the accumulation of fluid in the macula, leading to vision impairment. However, it typically occurs in response to conditions such as diabetic retinopathy, retinal vein occlusion, or Irvine-Gass syndrome, and is not directly related to a penetrating eye injury.

**Option C:** Optic nerve avulsion is a traumatic injury to the optic nerve, which can result from severe head trauma or surgical complications. While it can cause vision loss, it is not typically associated with a delayed onset or bilateral involvement.

**Option D:** Delayed vitreous hemorrhage can occur after a penetrating eye injury, but it typically presents with sudden onset of vision loss due to bleeding into the vitreous humor. It is not characterized by a gradual onset of bilateral symptoms.

**Clinical Pearl / High-Yield Fact**
Sympathetic ophthalmia is a rare but serious condition that requires prompt recognition and treatment to prevent vision loss. It is essential for ophthalmologists and emergency physicians to be aware of this condition, especially in patients with a history of penetrating eye injuries.

**Correct Answer Line**
✓ Correct Answer: B. Sympathetic ophthalmia