## Core Concept
Mercury poisoning primarily affects the nervous, gastrointestinal, and renal systems. The toxicity of mercury depends on its form, with elemental, inorganic, and organic mercury compounds having different effects. Mercury's mechanism of action involves binding to sulfhydryl groups in enzymes and proteins, disrupting cellular function.

## Why the Correct Answer is Right
The correct answer targets the **kidneys**. Mercury is known to cause renal damage, which can manifest as acute kidney injury or chronic kidney disease. The kidneys are particularly susceptible to mercury toxicity because they accumulate high levels of mercury, which can lead to necrosis of renal tubular cells and subsequent renal failure.

## Why Each Wrong Option is Incorrect
- **Option A:** While mercury does affect the nervous system, particularly with organic mercury compounds like methylmercury, which can cross the blood-brain barrier and cause neurological damage, the question seems to focus on a more direct and commonly emphasized effect.
- **Option B:** The gastrointestinal system can be affected by mercury poisoning, with symptoms including nausea, vomiting, and diarrhea, but this is not the most specifically targeted organ system in the context of mercury poisoning's hallmark effects.
- **Option C:** This option seems to relate to the nervous system, which, as mentioned, is indeed affected by mercury, particularly organic forms. However, the kidneys are a critical and commonly highlighted target.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **inorganic mercury** primarily affects the kidneys, leading to **renal tubular necrosis**, while **organic mercury** (like methylmercury) primarily affects the **central nervous system**. This distinction is crucial for understanding the varied presentations of mercury poisoning.

## Correct Answer Line
**Correct Answer: D. kidneys**