**Core Concept**
Porcelain jacket crowns are a type of dental restoration used to cover and protect teeth. They are typically indicated for teeth that are severely decayed, damaged, or discolored. However, their use is contraindicated in certain situations due to their design and construction.

**Why the Correct Answer is Right**
The correct answer is D. Excessive vertical overlap with little horizontal overlap. This is because porcelain jacket crowns are designed to be cemented onto the tooth, and an excessive vertical overlap can lead to difficulty in achieving a proper seal between the crown and the tooth. Additionally, the cement used to attach the crown may not be able to withstand the forces generated by the excessive vertical overlap, leading to potential complications such as crown loosening or fracture.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypoplastic teeth may actually be a good candidate for porcelain jacket crowns, as the crown can help to mask any discoloration or imperfections in the tooth.

**Option B:** Excessive horizontal overlap may not necessarily be a contraindication for porcelain jacket crowns, as the crown can still be designed and constructed to accommodate the overlapping teeth.

**Option C:** Decay is extensive but pulp is vital may actually be an indication for porcelain jacket crowns, as the crown can help to protect the tooth from further decay and preserve the pulp.

**Clinical Pearl / High-Yield Fact**
When considering porcelain jacket crowns, it's essential to assess the patient's occlusion and ensure that there is sufficient space for the crown to be properly seated and sealed. This can help to minimize the risk of complications and ensure a successful outcome.

**Correct Answer Line**
✓ Correct Answer: D. Excessive vertical overlap with little horizontal overlap.