**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children under the age of 5. It is a type of cancer that originates from the neuroblastic cells of the retina, which are responsible for the development of the visual pathway.

**Why the Correct Answer is Right**
Retinoblastoma is a genetically heterogeneous tumor, with both hereditary and sporadic forms. The hereditary form is caused by mutations in the RB1 gene, a tumor suppressor gene that regulates cell growth and division. Mutations in this gene lead to uncontrolled cell proliferation and tumor formation. The tumor suppressor function of the RB1 gene is crucial in preventing the development of retinoblastoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoblastoma is most commonly diagnosed in children under the age of 5. This statement is **true**.
**Option B:** The RB1 gene is a tumor suppressor gene that is mutated in retinoblastoma. This statement is **true**.
**Option C:** Retinoblastoma is a type of cancer that originates from the lens of the eye. This statement is **false**, as retinoblastoma originates from the retina, not the lens.
**Option D:** Retinoblastoma is a benign tumor. This statement is **false**, as retinoblastoma is a malignant tumor.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma is a classic example of a tumor suppressor gene mutation leading to cancer development. This concept is crucial in understanding the pathogenesis of various types of cancer, including breast, lung, and ovarian cancers, which are also associated with mutations in tumor suppressor genes.

**Correct Answer: C. Retinoblastoma is a type of cancer that originates from the lens of the eye.**