**Core Concept**
Pitting of direct filling gold is a common complication in dental restorations, particularly those involving direct gold fillings. It occurs due to the corrosion of the gold alloy, leading to the formation of pits or holes on the surface of the restoration.

**Why the Correct Answer is Right**
Pitting of direct filling gold is primarily caused by the oxidation of the gold alloy, resulting in the formation of a thin layer of gold oxide on the surface of the restoration. This layer is permeable to saliva and other fluids, allowing ions to diffuse through and react with the underlying metal, leading to corrosion and pitting. The presence of chloride ions, which are commonly found in saliva, can accelerate this process by reacting with the gold oxide to form a highly reactive compound that contributes to corrosion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pitting of direct filling gold is not primarily caused by mechanical factors such as excessive force or improper technique.
* **Option B:** This option is incorrect because pitting of direct filling gold is not primarily caused by bacterial contamination or infection.
* **Option C:** This option is incorrect because pitting of direct filling gold is not primarily caused by chemical factors such as exposure to certain chemicals or materials.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of pitting of direct filling gold, dentists should ensure that the gold alloy used is of high quality and that the restoration is properly cleaned and polished before placement. Additionally, the use of a protective layer or coating on the surface of the restoration can also help to prevent corrosion and pitting.

**Correct Answer:** D.