## **Core Concept**
The question describes a tooth with deep caries, pulpal involvement, and periapical radiolucency with a draining sinus into the gingival sulcus. This clinical presentation suggests a chronic infection of the dental pulp that has progressed to involve the periapical tissues, leading to the formation of a sinus tract.

## **Why the Correct Answer is Right**
The correct answer, **C. Chronic periapical (periradicular) cyst with sinus tract formation**, is right because a chronic periapical cyst is a type of odontogenic cyst that forms as a result of pulp necrosis and chronic periapical infection. The presence of a draining sinus into the gingival sulcus is a characteristic feature of a chronic periapical cyst that has formed a sinus tract to drain the infection. This condition often arises from a tooth with deep caries and pulpal involvement, leading to periapical radiolucency.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Acute periapical periodontitis does not typically present with a draining sinus into the gingival sulcus. It is an acute inflammatory response to pulp necrosis, characterized by pain and swelling.
- **Option B:** A periapical granuloma is a collection of granulation tissue that forms at the apex of a tooth in response to pulp necrosis. While it can present with periapical radiolucency, it is less likely to form a draining sinus compared to a chronic periapical cyst.
- **Option D:** A radicular cyst (also known as a periapical cyst) is similar to a chronic periapical cyst but is typically not described with sinus tract formation as a defining characteristic, although a sinus tract can occasionally form.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of a sinus tract in a tooth with deep caries and pulpal involvement often indicates a chronic infection process, such as a chronic periapical cyst. This condition may be asymptomatic except for the sinus tract, which can drain purulent material. Radiographically, a periapical radiolucency is often seen.

## **Correct Answer:** C. Chronic periapical (periradicular) cyst with sinus tract formation