A patient is suffering from pneumonia. Laboratory study shows acid-fast filamentous bacterium. The causative organism is –
**Core Concept**
The question is testing the student's ability to identify an acid-fast filamentous bacterium, which is a characteristic feature of certain types of infections. Acid-fast bacteria are those that resist decolorization by acid-alcohol during staining procedures, indicating the presence of mycolic acid in their cell walls.
**Why the Correct Answer is Right**
The correct answer is an acid-fast filamentous bacterium, specifically *Nocardia* species. These bacteria are known to cause nocardiosis, a type of infection that typically affects the lungs, but can also spread to other parts of the body. The filamentous nature of *Nocardia* species is due to their branching hyphae, which are visible under microscopy. The acid-fast property is a result of the presence of mycolic acid in their cell walls, making them resistant to decolorization.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because *Actinomyces* species are also filamentous bacteria, but they are not typically acid-fast. They are more commonly associated with actinomycosis, a type of infection that affects the oral cavity, lungs, or abdominal cavity.
* **Option B:** This option is incorrect because *Aspergillus* species are filamentous fungi, not bacteria. They are commonly associated with aspergillosis, a type of fungal infection that affects the lungs or other parts of the body.
* **Option C:** This option is incorrect because *Streptomyces* species are filamentous bacteria, but they are not typically acid-fast. They are more commonly associated with the production of antibiotics and are not typically pathogenic to humans.
**Clinical Pearl / High-Yield Fact**
* *Nocardia* species are typically found in soil and can be inhaled into the lungs, where they cause infection. The infection can progress to the bloodstream and affect other parts of the body, particularly in immunocompromised individuals.
**Correct Answer:** *Nocardia* species.