**Core Concept**
Leukokoria refers to an abnormal white reflection from the retina of the eye, often visible in low-light conditions. It is a significant sign that may indicate underlying ocular or systemic diseases.

**Why the Correct Answer is Right**
Leukokoria is typically seen in conditions where there is an abnormal accumulation of light or an obstruction in the visual pathway. This can be due to retinal detachment, cataracts, or tumors. In the case of retinoblastoma, leukokoria is often the presenting symptom due to the presence of a tumor in the retina. The tumor can cause an abnormal reflection of light, which appears as a white pupillary reflex.

**Why Each Wrong Option is Incorrect**
* **Option A:** Retinoblastoma - Leukokoria is a common presentation of retinoblastoma, making this an incorrect answer.
* **Option B:** Toxocariasis - This parasitic infection can cause leukokoria by inducing a retinal granuloma or by causing a retinal detachment. Therefore, this option is also incorrect.
* **Option C:** Leber's hereditary optic neuropathy - This condition typically presents with progressive loss of central vision in young adults, but it is not associated with leukokoria.

**Clinical Pearl / High-Yield Fact**
Leukokoria is often described as a "white pupillary reflex" or "cat's eye reflex." Remember that this sign can be a harbinger of serious ocular or systemic diseases, and prompt evaluation by an ophthalmologist is essential.

**Correct Answer:** D. Leber's hereditary optic neuropathy.