. A 55-year-old man who is being treated for adenocarcinoma of the lung is admitted to a hospital because of a temperature of 38.9degC (102degF), chest pain, and a dry cough. Sputum is collected. Gram’s stain of the sputum is unremarkable and culture reveals many small Gram-negative rods able to grow only on a charcoal yeast extract agar. This organism most likely is

**Core Concept**
The question tests the student's ability to identify a specific type of Gram-negative rod that can cause respiratory infections, particularly in immunocompromised patients. This organism is known for its unique growth requirements.

**Why the Correct Answer is Right**
The organism in question is likely *Legionella pneumophila*, a Gram-negative rod that is a common cause of community-acquired pneumonia, particularly in patients with underlying lung disease or immunosuppression. *Legionella* grows on charcoal yeast extract agar, which is a specialized medium that provides the necessary nutrients for its growth. The organism is typically identified by its ability to grow on this medium and by its characteristic "sulfur granules" seen on Gram stain. *Legionella* is a facultative intracellular pathogen that can cause severe pneumonia, and its identification is critical for appropriate treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the scenario described, as *Haemophilus influenzae* is a Gram-negative rod that grows on chocolate agar, not charcoal yeast extract agar.
* **Option B:** *Pseudomonas aeruginosa* is a Gram-negative rod that can cause respiratory infections, but it grows on a variety of media, including Pseudomonas agar and MacConkey agar.
* **Option C:** *Klebsiella pneumoniae* is a Gram-negative rod that can cause respiratory infections, but it grows on a variety of media, including MacConkey agar and blood agar.

**Clinical Pearl / High-Yield Fact**
*Legionella pneumophila* is a classic example of a pathogen that requires specialized media for growth, highlighting the importance of using the correct diagnostic tests for patients with suspected respiratory infections.

✓ Correct Answer: C. Legionella pneumophila