**Core Concept**
The most common cause of renal tumors in adults is a type of cancer that originates from the renal epithelium. Renal cell carcinoma (RCC) is the primary malignancy of the kidney, accounting for approximately 85% of all renal cancers.

**Why the Correct Answer is Right**
Renal cell carcinoma arises from the proximal convoluted tubule, a part of the nephron responsible for reabsorption and secretion. The exact mechanism of RCC is not well understood, but it is believed to involve genetic mutations in tumor suppressor genes such as VHL. The Von Hippel-Lindau (VHL) gene regulates the expression of hypoxia-inducible factors (HIFs), which are involved in angiogenesis and cell proliferation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Angiomyolipoma is a benign tumor of the kidney that is often associated with tuberous sclerosis complex. While it is a common renal tumor, it is not the most common cause of renal cancer in adults.
* **Option B:** Wilms tumor is a type of renal cancer that primarily affects children. It is a different entity from renal cell carcinoma and is not the most common cause of renal tumors in adults.
* **Option C:** Renal lymphoma is a rare type of cancer that affects the kidney. While it can occur in adults, it is not the most common cause of renal tumors.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate renal cell carcinoma from other renal masses, such as angiomyolipomas, cysts, and lymphoma, as the management and prognosis vary greatly among these entities.

**Correct Answer:** C. Renal cell carcinoma.