**Question:** Certain strains of Streptococcus mutans **are the cause of human dental caries by means of control studies**.

**Core Concept:** Streptococcus mutans is a bacterium that plays a significant role in the development of dental caries, which is tooth decay. The concept being tested here involves understanding how various strains of S. mutans contribute to caries formation through controlled studies.

**Why the Correct Answer is Right:**

Streptococcus mutans strains have been proven to be the cause of dental caries through control studies, which are experiments that isolate the effects of a single variable (in this case, the presence of S. mutans) while keeping other factors constant. This helps establish a cause-and-effect relationship. In the context of dental caries, these studies demonstrate the direct association between S. mutans and the development of tooth decay.

**Why Each Wrong Option is Incorrect:**

A. **Control studies** have shown the direct causality between S. mutans and dental caries, making this option incorrect.
B. While the production of insoluble polysaccharides by some strains of S. mutans plays a role in dental caries, it is not the primary reason for their involvement in caries development according to the question. This option is incorrect.
C. The bacterium Streptococcus mutans contributes to dental caries by producing acids, mainly lactic acid, through the fermentation of sugar. This option is partially correct but fails to address the role of control studies in establishing their causality.
D. Infective endocarditis is another infection caused by S. mutans, but this question focuses on dental caries, making this option incorrect.
E. While S. mutans can induce smooth surface caries, the question specifically asks about the role of control studies, making this option incorrect.

**Clinical Pearl:**

Understanding the role of Streptococcus mutans in dental caries is crucial for healthcare professionals. These bacteria are primarily responsible for the acid production through sugar fermentation, leading to demineralization of tooth enamel and subsequent decay. However, the importance of control studies in establishing their causality cannot be overemphasized. This knowledge helps in providing appropriate preventive measures, such as dental check-ups and regular dental care, to reduce the risk of dental caries.