**Core Concept**
Calculus, also known as tartar, is a hardened deposit on teeth that can lead to periodontal disease. The gingival tissues are sensitive to the presence of calculus, which can cause inflammation and damage to the surrounding tissues.

**Why the Correct Answer is Right**
Calculus can lead to periodontal disease by creating a barrier that prevents the gingival tissues from receiving essential nutrients and oxygen. The presence of calculus also allows for the accumulation of bacteria, which can lead to the production of virulence factors that damage the gingival tissues. The gingival crevice, also known as the sulcus, is particularly susceptible to the effects of calculus, as it is a narrow space where bacteria can easily accumulate and cause inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because calculus does not have a direct toxic effect on the gingival tissues. While calculus can lead to the accumulation of toxins, it is not the primary cause of damage to the gingival tissues.
**Option B:** This option is incorrect because calculus is not primarily composed of bacteria. While bacteria can accumulate on calculus, the primary component of calculus is inorganic minerals such as calcium and phosphate.
**Option C:** This option is incorrect because calculus does not have a direct effect on the pH of the gingival tissues. While calculus can lead to the accumulation of acid-producing bacteria, the primary cause of damage to the gingival tissues is the physical barrier created by the calculus.

**Clinical Pearl / High-Yield Fact**
The presence of calculus is a key indicator of periodontal disease, and regular removal of calculus is essential for maintaining good oral health. A thorough dental cleaning, also known as a prophylaxis, can help to remove calculus and prevent the development of periodontal disease.

**Correct Answer:** B.