**Core Concept**
The question describes a clinical presentation consistent with primary syphilis, caused by the bacterium *Treponema pallidum*. The patient's sexual history and physical examination findings, including the presence of a non-tender ulcer (chancre) with a firm, erythematous base and sharply demarcated borders, are characteristic of this disease.

**Why the Correct Answer is Right**
The correct answer is associated with the inflammatory process of granulomatous inflammation. This type of inflammation is a hallmark of syphilis, where the body's immune response attempts to contain the infection by forming granulomas. The presence of motile spirochetes on darkfield examination confirms the diagnosis of syphilis. The granulomatous inflammation in syphilis is characterized by the formation of granulomas, which are aggregates of immune cells, including macrophages, lymphocytes, and plasma cells, that attempt to wall off the infectious agent.

**Why Each Wrong Option is Incorrect**
* **Option A:** Suppurative inflammation is typically seen in bacterial infections, such as cellulitis or abscesses, which are not characteristic of syphilis.
* **Option B:** Caseous necrosis is a type of necrotizing inflammation seen in tuberculosis, which is not consistent with the presentation of primary syphilis.
* **Option D:** Coagulative necrosis is a type of necrotizing inflammation seen in ischemic injuries, such as myocardial infarction or cerebral infarction, which is not relevant to the clinical scenario described.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that syphilis can present in various stages, including primary, secondary, and tertiary syphilis, each with distinct clinical manifestations. The primary stage, as described in this question, is characterized by the presence of a chancre, which is a painless, non-tender ulcer.

**Correct Answer:** C.