**Core Concept**
The patient's symptoms suggest a sexually transmitted infection (STI) characterized by a primary lesion and regional lymphadenopathy. This clinical presentation is typical of a specific type of bacterial STI.

**Why the Correct Answer is Right**
The causative organism is Haemophilus ducreyi, which is responsible for chancroid. Chancroid is caused by the bacterium's ability to invade epithelial cells and cause necrotic lesions at the site of infection. The rubbery ulcer, also known as an ulceroglandular lesion, is a hallmark of chancroid, and the accompanying inguinal lymphadenopathy is a result of the body's immune response to the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** The causative organism of lymphogranuloma venereum (LGV) is Chlamydia trachomatis, not Haemophilus ducreyi. While LGV also presents with genital ulcers and lymphadenopathy, the clinical presentation and bacteriology differ from chancroid.
**Option B:** Treponema pallidum is the causative organism of syphilis, a different STI characterized by a painless chancre, not a rubbery ulcer.
**Option C:** Neisseria gonorrhoeae causes gonorrhea, which primarily manifests as a mucopurulent discharge, not a rubbery ulcer.
**Option D:** Herpes simplex virus (HSV) causes genital herpes, characterized by recurrent vesicular lesions, not a single rubbery ulcer.

**Clinical Pearl / High-Yield Fact**
Remember that chancroid is a reportable disease in many countries due to its potential for transmission and complications, such as bubonic lymphadenitis and genital fistulas.

**Correct Answer:** C. Haemophilus ducreyi.