**Core Concept**
Metal-ceramic restorations are a type of dental prosthesis that combines a metal substructure with a ceramic veneer. The bond between the metal and ceramic is crucial for the restoration's durability and longevity. Debonding, or the separation of the ceramic from the metal, is a common mechanical failure that can compromise the restoration's integrity.

**Why the Correct Answer is Right**
Debonding of the porcelain from the metal is the most common mechanical failure for metal-ceramic restorations due to the inherent differences in thermal expansion coefficients between the metal and ceramic materials. When the metal and ceramic are subjected to temperature changes, the ceramic expands and contracts more than the metal, leading to stress at the interface. Over time, this stress can cause the ceramic to separate from the metal, resulting in debonding. This phenomenon is particularly pronounced at the margin of the restoration, where the ceramic is most susceptible to thermal stress.

**Why Each Wrong Option is Incorrect**
**Option B:** Shrinkage of the porcelain at mouth temperature is not a common mechanical failure for metal-ceramic restorations. While the porcelain may shrink slightly during the firing process, this shrinkage is typically compensated for during the fabrication of the restoration. Moreover, the porcelain's shrinkage at mouth temperature is not a significant contributor to mechanical failure.

**Option C:** Debonding of the PFM (Porcelain-Fused-to-Metal) from the tooth is not a mechanical failure of the restoration itself, but rather a failure of the restoration to achieve a proper seal or bond with the tooth. This can be caused by various factors, including inadequate preparation of the tooth or a poorly fitting restoration.

**Option D:** None of the above is incorrect because debonding of the porcelain from the metal is indeed a common mechanical failure for metal-ceramic restorations.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of debonding, dentists should ensure that the metal substructure is properly prepared and that the ceramic veneer is bonded to the metal using a reliable luting agent. Additionally, regular follow-up appointments are crucial to detect any signs of debonding or other mechanical failures early on.

**✓ Correct Answer: A. Debonding of the porcelain from the metal.**