**Core Concept**
Renal cell carcinoma (RCC) is a type of kidney cancer with varying prognoses depending on several factors, including histological subtype, tumor size, stage, and patient characteristics. The prognosis of RCC is generally influenced by the tumor's aggressiveness and the patient's overall health status.

**Why the Correct Answer is Right**
The worst prognosis in RCC is associated with chromophobe RCC, which is a rare subtype accounting for approximately 5% of all RCC cases. Chromophobe RCC tends to have a more indolent course initially but can be more resistant to treatment, particularly in advanced stages. This subtype often lacks the aggressive features seen in clear cell RCC, such as high-grade nuclear atypia and vascular invasion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Clear cell RCC is the most common subtype (70-80% of cases), often presents with a more aggressive course, and typically has a poorer prognosis compared to chromophobe RCC. However, it is not the worst prognostic subtype.
* **Option B:** Papillary RCC is another subtype of RCC, often presenting with a more favorable prognosis than clear cell RCC. It accounts for approximately 10-15% of RCC cases and is typically associated with a better response to treatment.
* **Option C:** Collecting duct RCC is a rare and aggressive subtype, often presenting with a poor prognosis. However, it is not the worst prognostic subtype among RCC subtypes.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the prognosis of RCC is significantly influenced by the tumor's stage and histological subtype at diagnosis. Early detection and accurate histological classification are crucial for determining the optimal treatment strategy and predicting patient outcomes.

**Correct Answer: D. Chromophobe RCC**