**Core Concept**
Mercury is a toxic heavy metal that affects various organs, including the kidneys. The nephrotoxic effects of mercury are primarily due to its ability to accumulate in the renal cortex and damage the proximal convoluted tubules.

**Why the Correct Answer is Right**
Mercury exposure leads to the formation of reactive oxygen species, which cause oxidative stress and damage to the renal tubular cells. The proximal convoluted tubules are particularly susceptible to mercury toxicity due to their high metabolic rate and the presence of metallothionein, a protein that binds to mercury and enhances its nephrotoxicity. The resulting damage to the renal tubules can lead to acute kidney injury, characterized by hematuria, proteinuria, and electrolyte disturbances.

**Why Each Wrong Option is Incorrect**
**Option A:** The renal medulla is not primarily affected by mercury toxicity. While mercury can accumulate in the renal medulla, its nephrotoxic effects are more pronounced in the proximal convoluted tubules.
**Option B:** The renal pyramids are not directly affected by mercury toxicity. The pyramids are responsible for the concentration of urine, and mercury toxicity does not primarily impact this function.
**Option C:** The renal capsule is not directly involved in the nephrotoxic effects of mercury. The renal capsule is a fibrous layer that surrounds the kidney, and its primary function is to protect the kidney from external trauma.

**Clinical Pearl / High-Yield Fact**
Mercury toxicity can be prevented or minimized by avoiding exposure to mercury-containing compounds, such as thimerosal, and using protective equipment when handling mercury-containing products.

**Correct Answer:** A. Proximal convoluted tubules