**Core Concept**
Moisture contamination of amalgam during the restoration process can lead to a series of undesirable outcomes due to the chemical reaction between the mercury in the amalgam and the moisture. This reaction results in the formation of mercury vapor, which can be toxic to the patient and the dentist.

**Why the Correct Answer is Right**
The primary concern with moisture contamination is the release of mercury vapor, which can cause a range of health issues, including neurological problems and kidney damage. When amalgam comes into contact with moisture, the mercury reacts with the water to form a compound called mercuric oxide, which can be absorbed through the skin or inhaled as a vapor. This reaction is facilitated by the presence of certain enzymes, such as mercury reductase, which break down the mercury compounds into more toxic forms.

**Why Each Wrong Option is Incorrect**
* **Option A:** While moisture contamination can lead to mercury toxicity, it is not primarily responsible for the discoloration of the restoration.
* **Option B:** Moisture contamination does not directly cause the formation of galvanic currents between the amalgam and other restorative materials.
* **Option C:** While mercury toxicity is a concern, it is not the primary reason for the degradation of the restoration over time.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of mercury toxicity, dentists should take precautions when handling amalgam, such as wearing gloves and working in a well-ventilated area. Additionally, patients should be informed about the potential risks associated with amalgam restorations and provided with alternative options.

**Correct Answer: D. Mercury toxicity.**