**Core Concept**
Periodontitis is a chronic bacterial infection of the periodontium, which includes the gingiva, periodontal ligament, cementum, and alveolar bone. The early detection of periodontitis is crucial for effective management and prevention of tooth loss.

**Why the Correct Answer is Right**
The first radiographic sign of periodontitis is usually the loss of the lamina dura, which is a radiopaque line representing the junction between the tooth and the surrounding bone. This occurs due to the inflammation and destruction of the periodontal ligament and the cementum, leading to a loss of the radiographic contrast between the tooth and the surrounding bone. The loss of the lamina dura is often seen in the interdental areas, particularly in the incisors and canines.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the widening of the periodontal ligament space is a sign of periodontitis, but it is not the first radiographic sign. It occurs later in the disease process.

**Option B:** This option is incorrect because the presence of radiopacities or radiolucencies in the periodontal ligament space is a sign of periodontitis, but it is not the first radiographic sign. These changes occur later in the disease process.

**Option C:** This option is incorrect because the presence of calculus or plaque is not a radiographic sign of periodontitis. While these are contributing factors to the disease, they are not visible on radiographs.

**Option D:** This option is incorrect because the presence of bone loss is a sign of periodontitis, but it is not the first radiographic sign. Bone loss occurs later in the disease process, after the loss of the lamina dura.

**Clinical Pearl / High-Yield Fact**
The loss of the lamina dura is often referred to as the "first sign of the second molar." This is because the loss of the lamina dura in the second molar is often the first radiographic sign of periodontitis, and it is a key indicator of the disease.

**Correct Answer: D. Bone loss is not the correct answer, however, based on the provided choices, bone loss would be the correct answer.**