**Core Concept**
The patient presents with a genital ulcer, which is a type of cutaneous lesion that can be caused by various pathogens, including bacteria, viruses, and fungi. The history of unprotected sexual contact and the characteristics of the ulcer are crucial in determining the probable cause of infection.

**Why the Correct Answer is Right**
The patient's symptoms are highly suggestive of primary syphilis, a sexually transmitted infection caused by the bacterium *Treponema pallidum*. The painless, indurated, and partially healed genital ulcer, known as a chancre, is a classic presentation of primary syphilis. The presence of enlarged inguinal lymph nodes further supports this diagnosis. The bacterium *T. pallidum* enters the host through mucous membranes or minor skin abrasions, leading to localized inflammation and the formation of the chancre.

**Why Each Wrong Option is Incorrect**
* **Option A:** Herpes simplex virus (HSV) typically presents with painful vesicles or ulcers, not painless, indurated lesions. HSV is a common cause of genital ulcers, but the patient's symptoms do not align with this diagnosis.
* **Option B:** Chancroid, caused by *Haemophilus ducreyi*, typically presents with painful genital ulcers, often accompanied by lymphadenopathy. The patient's painless ulcer and absence of pain suggest a different diagnosis.
* **Option C:** Granuloma inguinale, caused by *Klebsiella granulomatis*, presents with beefy-red, ulcerative lesions that are often painless. However, the lesions are typically more extensive and not as circumscribed as in this case.

**Clinical Pearl / High-Yield Fact**
Remember the "3 Ps" of primary syphilis: painless, progressive, and persisting. The patient's symptoms are a classic example of this presentation, and it is essential to consider syphilis in the differential diagnosis of genital ulcers, especially in the context of a high-risk sexual history.

**Correct Answer:** C.