**Core Concept**
The eruption of the first tooth in infants is a significant milestone in dental development. This process is influenced by various factors, including genetics, nutrition, and overall health.

**Why the Correct Answer is Right**
The first tooth to appear is typically the two lower central incisors, also known as the lower front teeth. This occurs due to the development of the dental lamina, a layer of epithelial cells that gives rise to the teeth. The lower central incisors are the first teeth to erupt because they are the most anterior (front) and are subjected to the least amount of pressure from the surrounding tissues. The eruption of these teeth is usually accompanied by a visible bulge in the gum tissue, which eventually breaks through to form the tooth.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the first molars are not the first teeth to erupt. They typically appear between 12-16 months of age, much later than the lower central incisors.
* **Option B:** This option is incorrect because the upper central incisors are not the first teeth to appear. They usually erupt after the lower central incisors, around 6-10 months of age.
* **Option C:** This option is incorrect because the canines (cuspids) are not the first teeth to erupt. They typically appear between 16-22 months of age and are located more laterally (sideways) than the lower central incisors.

**Clinical Pearl / High-Yield Fact**
Remember that the eruption of the first tooth is a significant milestone in dental development, and any significant delay or absence of tooth eruption should prompt further evaluation by a pediatric dentist or primary care physician to rule out underlying conditions that may be affecting dental development.

**Correct Answer: D. Lower central incisors. The first tooth to appear is the lower central incisors, typically around 6 months of age.**