A 40-year-old otherwise healthy gardener presents with several subcutaneous nodules on his right hand, where he had cut himself on rose thorns. Physical exam reveals several erythematous fluctuant lesions. Which of the following organisms is most likely responsible for his condition?

**Core Concept**
The patient's presentation of subcutaneous nodules and erythematous fluctuant lesions following a puncture wound from rose thorns is suggestive of a bacterial infection caused by an organism commonly found in soil and plant material.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with a diagnosis of **anthrax**, caused by the gram-positive bacterium **Bacillus anthracis**. This organism is typically found in soil and can infect animals, including humans, through cuts or abrasions in the skin. The spores of B. anthracis can survive for long periods in the environment and can be inhaled, ingested, or enter the body through breaks in the skin. The infection can progress to a severe form of anthrax, characterized by the formation of subcutaneous nodules, known as **eschars**, which can eventually rupture and form ulcers.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is a common cause of skin infections, but it typically presents with abscesses or cellulitis, rather than the characteristic subcutaneous nodules and eschars seen in anthrax.
* **Option B:** Pseudomonas aeruginosa is a gram-negative bacterium that can cause skin infections, especially in immunocompromised individuals, but it is not typically associated with puncture wounds from plant material.
* **Option D:** Erysipelothrix rhusiopathiae is a gram-positive bacterium that can cause erysipeloid, a skin infection typically seen in individuals handling animals or animal products, but it does not typically present with subcutaneous nodules.

**Clinical Pearl / High-Yield Fact**
Anthrax can be prevented by proper wound care, including immediate cleaning and dressing of the wound, as well as prophylactic antibiotics in high-risk individuals.

**Correct Answer:** C. Bacillus anthracis.

✓ Correct Answer: D. Sporothrix