**Core Concept**
Galvanic shock, also known as electrolytic shock, occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte, such as saliva. This phenomenon is a classic cause of transient, shooting pain in patients with dental restorations. In the context of a gold onlay, galvanic shock can occur when the gold restoration occludes with another metal restoration in the mouth.

**Why the Correct Answer is Right**
The correct answer is D. Galvanic current caused by gold onlay occluding with a large restoration. When the gold onlay comes into contact with a large metal restoration, such as an amalgam filling, the difference in electrical potential between the two metals generates a small electric current. This electric current, or galvanic shock, stimulates the nerves in the pulp of the tooth, causing a sharp, shooting pain. The presence of saliva in the mouth acts as an electrolyte, facilitating the flow of electric current between the two metals.

**Why Each Wrong Option is Incorrect**
**Option A:** Supraocclusion of restoration refers to the occlusion of the restoration above the occlusal plane, which may cause discomfort or pain due to excessive pressure on the tooth or surrounding tissues. However, it does not explain the transient, shooting pain associated with galvanic shock.

**Option B:** Retained cement in the gingival sulcus can cause pain and discomfort due to irritation of the surrounding tissues. However, it does not account for the characteristic shooting pain associated with galvanic shock.

**Option C:** Excess acid in the cement mix can cause irritation of the pulp or surrounding tissues, leading to pain. However, it does not explain the specific phenomenon of galvanic shock.

**Clinical Pearl / High-Yield Fact**
To avoid galvanic shock in patients with dental restorations, it is essential to ensure that all metal restorations are properly isolated from each other using a suitable material, such as a rubber dam or a zinc oxide-eugenol cement. This can help prevent the generation of an electric current between the two metals.

**✓ Correct Answer: D. Galvanic current caused by gold onlay occluding with a large restoration**