**Core Concept:** Rb gene is a tumor suppressor gene that plays a crucial role in regulating cell cycle progression and preventing uncontrolled cell growth. Mutations in the Rb gene lead to loss of function and increased cell proliferation, predisposing an individual to develop certain cancers.

**Why the Correct Answer is Right:** In this scenario, the patient with a known mutation in the 'Rb gene' is disease-free from Retinoblastoma, which is a childhood eye cancer. The Rb gene mutation predisposes the patient to develop a malignancy due to loss of tumor suppressor function. Given the Rb gene's role in regulating cell cycle progression, an individual with an Rb gene mutation has a higher likelihood of developing malignancies involving rapid cell growth and uncontrolled cell proliferation.

**Why Each Wrong Option is Incorrect:**

A. Leukemia: While Rb gene mutations can lead to increased risk of certain cancers, leukemia is not primarily associated with Rb gene mutations.

B. Lung Cancer: This answer choice is incorrect because Rb gene mutations primarily increase the risk of developing retinoblastoma and other solid tumors, not lung cancer.

C. Lymphoma: Similar to option A, lymphomas are not predominantly linked to Rb gene mutations.

D. Melanoma: While Rb gene mutations may contribute to an increased risk of melanoma, the primary focus is on solid tumors and eye cancer, not melanoma.

**Clinical Pearl:** Rb gene mutations are responsible for an elevated risk of developing solid tumors and eye cancer, specifically retinoblastoma. It is essential for healthcare professionals to monitor such patients closely for early detection and intervention of potential malignancies.

**Correct Answer:** C. Lymphoma