Okay, let's tackle this question. The patient has a single kidney, which is important because any treatment must consider preserving renal function. The mass is exophytic and 4 cm at the lower pole. So, what's the best management?

First, the core concept here is the management of renal masses in patients with a solitary kidney. The key is to balance tumor removal with preserving as much renal tissue as possible. Since the patient only has one kidney, we can't afford to remove too much, so conservative options are preferred.

The correct answer is likely partial nephrectomy. That's because it allows removal of the tumor while preserving the rest of the kidney. For a 4 cm tumor, partial nephrectomy is feasible, especially if it's localized and exophytic, making surgical access easier. Radical nephrectomy would be too risky here due to the lack of a second kidney.

Now, looking at the options. Let's assume the options are A: Radical nephrectomy, B: Ablation, C: Partial nephrectomy, D: Observation. The wrong options would be:

A: Radical nephrectomy is incorrect because removing the only kidney would lead to end-stage renal disease. So that's not an option here.

B: Ablation might be considered, but for a 4 cm tumor, ablation (like cryoablation or RFA) has lower success rates compared to partial nephrectomy. Also, the exophytic location might make ablation less effective or more technically challenging.

D: Observation is not appropriate for a 4 cm renal mass. The size is above the threshold (usually 4 cm) where active surveillance is recommended, so intervention is necessary.

The clinical pearl here is that in patients with a solitary kidney, partial nephrectomy is the gold standard for localized renal masses to preserve renal function. The key is to avoid losing the only functioning kidney. Remember, the NCCN guidelines support this approach for tumors up to 7 cm, so a 4 cm is definitely within that range.

So, the correct answer is C: Partial nephrectomy.

**Core Concept**
This question tests the management of renal masses in patients with a solitary kidney. The primary goal is to remove the tumor while preserving renal function, as radical nephrectomy would lead to end-stage renal disease in such patients. Exophytic tumors (growing outward) are often amenable to partial nephrectomy due to better surgical access.

**Why the Correct Answer is Right**
**Partial nephrectomy** is the standard treatment for localized renal cell carcinoma in patients with a solitary kidney. It allows precise excision of the tumor with a margin of healthy tissue while preserving the majority of renal parenchyma. For a 4 cm exophytic mass, the tumor is small enough to be safely resected, and the exophytic growth pattern facilitates surgical dissection. This approach minimizes the risk of postoperative chronic kidney disease, which is critical in solitary kidney cases.

**Why Each Wrong Option is Incorrect**
**Option A: Radical nephrectomy** – Incorrect because it removes the entire kidney, leaving the patient without renal function, which is unacceptable in a solitary kidney scenario.
**Option B: Radiofrequency ablation** – Incorrect because it is less effective for tumors >3 cm and carries a higher risk of incomplete ablation in exophytic lesions

✓ Correct Answer: A. Paial Nephrectomy