**Core Concept**
The patient's symptoms suggest a condition where vision is normal, but there is discomfort or pain during visual tasks, which is not relieved by occluding one eye. This condition is related to the binocular vision system and the fusion of images from both eyes.

**Why the Correct Answer is Right**
The patient's symptoms are characteristic of convergence insufficiency, a condition where the eyes have difficulty converging (turning inward) to focus on a near object. This results in eye strain and discomfort during prolonged near work, such as reading. The patient's normal vision and normal examination findings in both eyes, including 6/5 Snellen's reading, rule out other conditions such as amblyopia or strabismus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the patient's symptoms, as the patient's vision is normal, and there is no indication of a refractive error.
* **Option B:** This option is incorrect because convergence insufficiency is not related to the presence of a squint or strabismus, which is a misalignment of the eyes.
* **Option C:** This option is incorrect because convergence insufficiency is not related to the presence of a cataract or other ocular pathology.

**Clinical Pearl / High-Yield Fact**
Convergence insufficiency is a common condition in children and adolescents, particularly during the pre-teen years, and is often associated with prolonged near work, such as reading or computer use.

**Correct Answer: A. Convergence insufficiency**