**Core Concept**
Wilms' tumor, also known as nephroblastoma, is a type of kidney cancer primarily affecting children. The prognosis of Wilms' tumor depends on several factors, including the tumor's stage, histology, and genetic characteristics.

**Why the Correct Answer is Right**
The most important prognostic factor of Wilms' tumor is the presence of anaplasia, which refers to the presence of abnormal cells that are resistant to chemotherapy and radiation. Anaplasia is associated with a higher risk of recurrence and metastasis, and it is considered a high-risk feature. The presence of anaplasia is typically assessed through histopathological examination of the tumor tissue. The International Society of Paediatric Oncology (SIOP) and the Children's Oncology Group (COG) have established guidelines for the treatment of Wilms' tumor, which take into account the presence of anaplasia.

**Why Each Wrong Option is Incorrect**
**Option A:** Histological subtype is also an important prognostic factor in Wilms' tumor. However, it is not the most important factor. The presence of anaplasia is a more significant predictor of outcome.
**Option B:** Tumor stage is a critical factor in the management of Wilms' tumor, but it is not the most important prognostic factor. The presence of anaplasia is a more significant predictor of outcome.
**Option C:** Genetic abnormalities, such as mutations in the WT1 gene, are associated with Wilms' tumor. However, they are not the most important prognostic factor. The presence of anaplasia is a more significant predictor of outcome.
**Option D:** Patient age is not a significant prognostic factor in Wilms' tumor.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that anaplasia is a high-risk feature in Wilms' tumor, and its presence requires aggressive treatment. The SIOP and COG guidelines should be followed closely in the management of Wilms' tumor.

**Correct Answer:** C. Anaplasia.