**Core Concept**
PFM (Porcelain Fused to Metal) restorations are a type of dental prosthesis that combines the aesthetic properties of porcelain with the strength and durability of metal. The main disadvantage of PFM restorations is related to the metal-porcelain interface, which can be a source of clinical problems.

**Why the Correct Answer is Right**
The main disadvantage of PFM restorations is the galvanic shock that can occur when two dissimilar metals are in contact with each other in the oral environment. This can lead to a sudden release of ions, causing pain and potentially damaging the surrounding tissues. The metal-porcelain interface can also be a site for corrosion, which can weaken the restoration and lead to its failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the specific issue with PFM restorations.
* **Option B:** This option is incorrect because it is a benefit of PFM restorations, not a disadvantage.
* **Option C:** This option is incorrect because it is a potential advantage of PFM restorations, not a disadvantage.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient for PFM restoration, it is essential to consider the potential for galvanic shock and corrosion at the metal-porcelain interface. This can be mitigated by using a noble metal alloy and ensuring proper isolation and placement of the restoration.

**Correct Answer: D. Galvanic shock and corrosion at the metal-porcelain interface.**