**Core Concept**
Buruli ulcer is a chronic, progressive skin infection caused by a nontuberculous mycobacterium that primarily affects the skin and subcutaneous tissues. The causative agent is *Mycobacterium ulcerans*, which produces a unique toxin called mycolactone, leading to tissue necrosis and ulceration.

**Why the Correct Answer is Right**
*Mycobacterium ulcerans* is the sole organism responsible for Buruli ulcer, a tropical disease prevalent in West Africa. It spreads through environmental exposure (e.g., soil, water), and the infection leads to painless, slow-developing ulcers with extensive tissue destruction. Mycolactone, a lipophilic toxin produced by *M. ulcerans*, inhibits protein synthesis and causes severe local tissue damage and immune suppression.

**Why Each Wrong Option is Incorrect**
Option A: *M. marinum* causes tuberculosis-like infections in immunocompromised patients and is associated with tuberculous lymphadenitis or cutaneous infections, but not Buruli ulcer.
Option C: *M. kansasii* is a non-tuberculous mycobacterium causing pulmonary or extrapulmonary disease, but not Buruli ulcer.
Option D: *M. smegmatis* is a saprophytic, fast-growing mycobacterium commonly used in research and does not cause skin ulcers.

**Clinical Pearl / High-Yield Fact**
Buruli ulcer is a public health concern in tropical regions; early diagnosis is crucial because the disease progresses rapidly and can lead to severe deformities. The presence of a painless, indurated ulcer with a "punched-out" appearance is a key clinical clue.

✓ Correct Answer: B. M. Ulcerans