**Core Concept**
Retinoblastoma is a malignant tumor of the retina that can spread through the optic nerve and vitreous humor, leading to metastasis. This concept is crucial in understanding the clinical behavior of the tumor and the need for prompt treatment.

**Why the Correct Answer is Right**
The primary route of spread for retinoblastoma is through the optic nerve, which is a direct extension of the tumor. This occurs due to the continuity of the optic nerve with the retina, allowing the tumor cells to invade and spread along the nerve fibers. The tumor can also spread through the vitreous humor, a clear gel-like substance that fills the center of the eye, which can lead to seeding of tumor cells on the surface of the retina and optic nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may seem plausible, but the primary route of spread for retinoblastoma is not through the bloodstream.
**Option B:** While lymphatic spread is a common feature of many cancers, retinoblastoma primarily spreads through the optic nerve and vitreous humor.
**Option C:** This option is incorrect as the spread through the optic nerve is a direct extension of the tumor, not a result of local invasion.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma is a highly malignant tumor that can spread rapidly through the optic nerve, emphasizing the need for prompt diagnosis and treatment to prevent metastasis.

**Correct Answer:** C.