The maximum rate of eruption, as the tooth cusps reach the surface epithelium is around ___________ μm/day.

**Core Concept**
The process of tooth eruption involves the gradual movement of the tooth through the alveolar bone and surrounding tissues. It is influenced by various factors, including the rate of bone resorption, the formation of a periodontal ligament, and the pressure exerted by the tooth on the surrounding tissues.

**Why the Correct Answer is Right**
The rate of tooth eruption is primarily determined by the rate of bone resorption. As the tooth cusps reach the surface epithelium, the rate of eruption is maximum. This is because the bone resorption is more rapid than the formation of a periodontal ligament, allowing the tooth to move more quickly through the alveolar bone. The rate of eruption also depends on the size of the tooth, with larger teeth erupting more slowly.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely too low, as the rate of eruption is typically in the range of 0.5-2.5 mm/day, which is equivalent to 500-2500 μm/day.
* **Option B:** This option is likely too high, as the rate of eruption is not typically as rapid as this value would suggest.
* **Option C:** This option is likely too low, as the rate of eruption is typically in the range of 0.5-2.5 mm/day, which is equivalent to 500-2500 μm/day.

**Clinical Pearl / High-Yield Fact**
The rate of tooth eruption can be influenced by various factors, including genetics, nutrition, and systemic diseases. For example, children with rickets may experience delayed tooth eruption due to impaired bone mineralization.

**Correct Answer: C. 2-4 μm/day.**