**Core Concept**
The etiology of periodontitis is multifactorial, involving a combination of local and systemic factors. However, the primary cause of periodontitis is the accumulation of dental plaque, a biofilm composed of bacteria, which leads to chronic inflammation and destruction of the periodontal tissues.

**Why the Correct Answer is Right**
Local irritating factors, specifically the accumulation of dental plaque and calculus, trigger an immune response in the periodontal tissues, leading to the release of pro-inflammatory mediators and the activation of various enzymes involved in tissue destruction. The bacteria in the plaque produce toxins and enzymes that degrade the extracellular matrix, causing the loss of attachment and supporting structures of the teeth. This is a classic example of a local irritant causing a systemic response, leading to the development of periodontitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Occlusal trauma, while it can contribute to periodontal disease, is not the primary etiology. It can exacerbate existing periodontal disease but is not a common cause.
**Option B:** Systemic factors, such as diabetes, smoking, and certain medications, can contribute to the progression of periodontitis but are not the primary cause.
**Option D:** Hormonal defects, such as those seen in pregnancy or hormonal imbalances, can affect the periodontal tissues but are not a common etiology of periodontitis.

**Clinical Pearl / High-Yield Fact**
The accumulation of dental plaque is the primary cause of periodontitis, and regular dental hygiene practices, such as brushing and flossing, can prevent or control the disease. This highlights the importance of patient education and regular dental check-ups in maintaining oral health.

**✓ Correct Answer: C. Local irritating factors**