**Core Concept**
Nocardia and Actinomyces are both filamentous, gram-positive bacteria that are morphologically similar, making their differentiation challenging. However, they have distinct immunological and biochemical properties, which can be exploited for their identification.

**Why the Correct Answer is Right**
Nocardia can be differentiated from Actinomyces by using the Ziehl-Neelsen (ZN) stain, also known as acid-fast staining. Nocardia species are partially acid-fast, meaning they resist decolorization with acid-alcohol, whereas Actinomyces species are not acid-fast. This is due to the presence of mycolic acids in the cell walls of Nocardia, which impart acid-fastness. The ZN stain is a crucial tool in the identification of Nocardia, especially in clinical specimens.

**Why Each Wrong Option is Incorrect**
**Option A:** Gram stain is not sufficient for differentiating Nocardia from Actinomyces, as both appear as gram-positive, filamentous rods. While gram staining can provide some clues, it is not a reliable method for their identification.

**Option C:** While it is true that Nocardia can cause mycetoma, a chronic granulomatous infection of the skin and subcutaneous tissues, Actinomyces can also cause similar infections, such as actinomycosis. This option is not a reliable criterion for differentiating the two organisms.

**Option D:** Nocardia is actually an aerobic bacterium, not a facultative anaerobe. Facultative anaerobes can grow in both aerobic and anaerobic conditions, whereas Nocardia requires oxygen for growth. This option is incorrect and misleading.

**Clinical Pearl / High-Yield Fact**
Nocardia infections can be challenging to diagnose, especially in immunocompromised patients. The use of acid-fast staining, such as the ZN stain, is a critical step in identifying Nocardia in clinical specimens. This can help guide appropriate treatment and prevent delayed or inadequate therapy.

**✓ Correct Answer: B. ZN stain**