**Core Concept:** Avulsion is the complete removal of a tooth from its alveolar socket due to trauma or accident. In such cases, the tooth needs to be replanted as soon as possible to increase the chances of survival. Prolonged extra-oral time refers to the duration the tooth is out of the oral cavity before replantation.

**Why the Correct Answer is Right:** The best medium for storing an avulsed tooth during prolonged extra-oral periods is **Dry Ice (Solid Carbon Dioxide)**, commonly known as liquid nitrogen. Dry ice is a reliable and efficient way to preserve the tooth's viability due to its extremely low temperatures (-78.5°C/-109.3°F). At these temperatures, Dry Ice effectively slows down cellular activity, preventing the tooth from drying out or dying.

**Why Each Wrong Option is Incorrect:**

A. **Aluminium Foam (Aluminium Hydroxide)** is incorrect because it is not a reliable medium for preserving the tooth's viability due to its insulating properties. At room temperature, it does not lower the temperature significantly, which would impair the tooth's survival.

B. **Human Saliva (Salivary Amylase)** is incorrect because it is not a sterile environment, which increases the risk of contamination and infection. Furthermore, saliva contains enzymes that can break down tooth structures, reducing the tooth's chances of survival.

C. **Human Blood (Coombs Test)** is incorrect because human blood is not suitable for tooth preservation due to its potential to cause blood clotting, which complicates replantation and increases the risk of infection. Additionally, blood contains clotting factors that can be detrimental to the tooth's survival.

E. **Plain Ice (Water)** is incorrect because plain ice is not an effective medium for preserving a tooth's viability due to its relatively higher temperature compared to Dry Ice. At temperatures near the freezing point, ice can cause cellular damage and reduce the tooth's chances of survival.

**Clinical Pearl:** Prolonged extra-oral time reduces the chances of a tooth's survival. The correct storage medium is crucial for successful replantation. Immediate replantation is ideal, but in clinical practice, circumstances may dictate the use of an appropriate storage medium to preserve the tooth for replantation.