## **Core Concept**
The question tests knowledge of microbiological identification techniques, specifically acid-fast staining. Acid-fast organisms retain the stain carbol fuchsin after decolorization with an acid-alcohol solution. The use of 1% sulphuric acid as a decolorizer is less common but indicates the organism has a high degree of acid-fastness.

## **Why the Correct Answer is Right**
The organism in question is described as filamentous and acid-fast with 1% sulphuric acid. *Nocardia* species are aerobic, Gram-positive, filamentous bacteria that are partially acid-fast due to their cell wall composition, which contains mycolic acids. This characteristic makes them susceptible to acid-alcohol decolorization but resistant to 1% sulphuric acid.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Streptomyces* species are filamentous, but they are not typically acid-fast. They belong to the Actinomycetota and are more commonly associated with soil and antibiotic production.
- **Option B:** *Actinomyces* species are filamentous, but they are not acid-fast. They are anaerobic and commonly found in the oral cavity and gastrointestinal tract.
- **Option D:** *Mycobacterium* species are acid-fast but are not typically described as filamentous; they are usually rod-shaped.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that *Nocardia* infections are more common in immunocompromised individuals and can cause a range of diseases, including pulmonary nocardiosis, brain abscesses, and skin infections. Nocardia is a classic example of a partially acid-fast organism.

## **Correct Answer:** C. Nocardia.