## Core Concept
The question revolves around a common complication following endodontic treatment and restoration of a tooth with a cast post-and-core and a metal-ceramic crown. The key issue here is the patient's complaint of pain, especially on biting, three months post-restoration, with normal radiographic findings and tooth mobility tests. This scenario suggests a mechanical or procedural issue related to the restoration rather than a pathological process like infection or periodontal disease.

## Why the Correct Answer is Right
The correct answer, **C. Occlusal trauma due to high restoration**, is the most probable cause of pain in this scenario. When a tooth is restored with a cast post-and-core and a metal-ceramic crown, it's crucial that the occlusal surface of the restoration is properly aligned with the opposing dentition to avoid excessive occlusal forces. If the restoration is too high, it can lead to occlusal trauma, causing pain, especially on biting. This condition occurs because the tooth is subjected to forces beyond its adaptive capacity, leading to inflammation of the periodontal ligament and pain.

## Why Each Wrong Option is Incorrect
- **Option A:** While a **cracked tooth** can indeed cause pain, especially on biting, the scenario provided does not offer specific clues like a history of trauma or symptoms that would specifically point towards a cracked tooth, such as pain on release of biting pressure.
- **Option B:** **Incomplete removal of caries** is not directly suggested by the symptoms or the provided history. This option does not correlate well with pain on biting in the context of a recently restored tooth with normal radiographic findings.
- **Option D:** **Post-and-core loosening** could potentially cause pain but would likely present with other symptoms or radiographic signs not mentioned, such as mobility of the post or changes in the surrounding bone.

## Clinical Pearl / High-Yield Fact
A critical point to remember is that occlusal trauma can occur due to high restorations, and this is a common and often easily remediable cause of post-restorative pain. Checking the occlusion and adjusting high points is a simple yet crucial step in managing such cases.

## Correct Answer Line
**Correct Answer: C. Occlusal trauma due to high restoration.**