**Core Concept**
Primary syphilis is typically characterized by a painless, indurated ulcer known as a chancre, which usually occurs at the site of exposure. This condition is caused by the bacterium *Treponema pallidum*.

**Why the Correct Answer is Right**
The diagnosis of primary syphilis is made based on the clinical presentation, including the presence of a painless indurated ulcer with everted margins, typically at the site of exposure. The chancre is usually firm to the touch and may be accompanied by regional lymphadenopathy. The presence of the chancre is a key diagnostic feature of primary syphilis, and its location is often consistent with the history of sexual exposure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option does not describe a common presentation of primary syphilis. A painless, indurated ulcer is characteristic of primary syphilis, whereas a painless, bleeding ulcer is more suggestive of a different condition.
* **Option B:** Herpes simplex virus (HSV) typically presents with multiple painful vesicles or ulcers, not a single painless indurated ulcer.
* **Option C:** Chancroid, caused by *Haemophilus ducreyi*, typically presents with one or more painful ulcers, which is not consistent with Kallu's presentation.
* **Option D:** This option is not a valid diagnosis for the described clinical presentation.

**Clinical Pearl / High-Yield Fact**
The chancre of primary syphilis is a classic example of a "tender" lymphadenopathy, where the lymph nodes are painful on palpation. This can be a helpful distinguishing feature from other causes of lymphadenopathy.

**Correct Answer:** C.