A child is born with a single functional copy of a tumor suppressor gene. At the age of 5 years, the remaining normal allele is lost through mutation. As a result, the ability to control the transition from G1 to the S phase of the cell cycle is lost. Which of the following neoplasms is most likely to arise by means of this mechanism-

**Core Concept**
The question is testing the understanding of the relationship between tumor suppressor genes and the cell cycle, specifically the role of the p53 gene in controlling the transition from G1 to the S phase. Tumor suppressor genes, such as TP53, encode for proteins that regulate cell growth and division, preventing uncontrolled cell proliferation and tumor formation.

**Why the Correct Answer is Right**
The loss of a functional p53 gene leads to the accumulation of mutations in the genome, as the normal G1-S checkpoint is disrupted. This allows cells with damaged DNA to enter the S phase and replicate, increasing the likelihood of further mutations and cancer development. The p53 protein acts as a tumor suppressor by preventing cells with damaged DNA from dividing, thereby preventing the formation of tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the mechanism described in the question. While it is true that mutations in the RB1 gene can lead to retinoblastoma, it is not the most likely neoplasm to arise from the loss of p53 function.

**Option B:** This option is incorrect because the loss of p53 function is more closely associated with Li-Fraumeni syndrome, a condition characterized by an increased risk of developing multiple types of cancer, rather than a single neoplasm.

**Option C:** This option is incorrect because the loss of p53 function is not directly related to the development of neuroblastoma.

**Option D:** This option is incorrect because the loss of p53 function is more closely associated with Wilms tumor, a type of kidney cancer, rather than hepatoblastoma.

**Clinical Pearl / High-Yield Fact**
The p53 gene is often referred to as the "guardian of the genome" due to its role in preventing the propagation of cells with damaged DNA. Mutations in the p53 gene are found in approximately 50% of human cancers, highlighting the importance of this gene in maintaining genomic stability.

**Correct Answer:** D.

✓ Correct Answer: A. Retinoblastoma