Nocardia is differentiated from Actinomyces by ?
**Core Concept**
Nocardia and Actinomyces are both filamentous, gram-positive bacteria that can cause infections in humans. However, they belong to different genus and exhibit distinct morphological and biochemical characteristics.
**Why the Correct Answer is Right**
Nocardia is differentiated from Actinomyces by its ability to form a tough, non-acid fast, filamentous structure called a mycelium. This mycelium is composed of branching filaments that can penetrate deep into tissues. In contrast, Actinomyces forms short, non-branching filaments that are generally limited to the surface of tissues. The presence of mycelium in Nocardia is a key distinguishing feature from Actinomyces.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the characteristic that differentiates Nocardia from Actinomyces.
**Option B:** This option is incorrect as it is a characteristic of Actinomyces, not Nocardia.
**Option C:** This option is incorrect as it refers to a characteristic of both Nocardia and Actinomyces, and does not provide a clear distinction between the two.
**Clinical Pearl / High-Yield Fact**
Nocardia infections can be difficult to diagnose, but the presence of a mycelium on tissue Gram stain or culture is a key diagnostic clue. It is essential to consider Nocardia in the differential diagnosis of chronic infections in immunocompromised patients.
**Correct Answer: D. Presence of mycelium.**