**Core Concept:**
Actinomycosis is a rare but treatable bacterial infection caused by the Gram-positive, filamentous bacteria Actinomyces species. Actinomycetes produce sulfur granules, which are the distinctive pathognomonic feature of the disease, helping in its diagnosis. These sulfur granules are composed of sulfur, calcium salts, and organic material.

**Why the Correct Answer is Right:**
The correct answer is **D.** Actinomyces israelii is the most predominant constituent of sulfur granules in Actinomycosis. This bacterium is part of the normal oral flora and can cause extra-oral and intra-oral infections, leading to the formation of characteristic sulfur granules. These granules are composed of sulfur, calcium salts, and organic material, which are produced as a response to the bacteria's extracellular polysaccharide capsule.

**Why Each Wrong Option is Incorrect:**
A. Actinomyces naeslundii is not the predominant constituent of sulfur granules in Actinomycosis. Although it is an oral bacterium, it is less commonly associated with Actinomycosis infections compared to A. israelii.

B. Actinomyces gerencseriae is another oral bacterium associated with Actinomycosis. However, it is less commonly associated with Actinomycosis infections compared to A. israelii.

C. Actinomyces meyerii is an oral bacterium, but it is less commonly associated with Actinomycosis infections compared to A. israelii and A. naeslundii.

**Clinical Pearl:**
Since the correct answer is D (Actinomyces israelii), it is essential for clinicians to consider Actinomycosis in the differential diagnosis of chronic granulomatous infections, especially in immunocompromised patients. Early diagnosis and appropriate antibiotic therapy can lead to successful treatment outcomes.