**Core Concept**
The question tests the student's understanding of acid-fastness, a property exhibited by certain bacteria and protozoa. Acid-fastness is a characteristic that allows these microorganisms to resist decolorization during the staining process, typically with carbol fuchsin and acid-alcohol. This property is often associated with the presence of mycolic acid in the cell wall of mycobacteria and some other actinomycetes.

**Why the Correct Answer is Right**
Fusobacterium nucleatum is a gram-negative, non-spore-forming, anaerobic rod that does not possess the characteristic mycolic acid-rich cell wall structure. As a result, it does not exhibit acid-fastness and cannot resist decolorization during staining. This is in contrast to the other options, which are known to display acid-fastness due to their unique cell wall composition.

**Why Each Wrong Option is Incorrect**
**Option A:** Nocardia is an actinomycete that exhibits acid-fastness due to the presence of mycolic acid in its cell wall. It is a gram-positive, partially acid-fast bacterium that can cause infections in immunocompromised individuals.
**Option C:** Rhodococcus is another actinomycete that displays acid-fastness, similar to Nocardia. It is a gram-positive, partially acid-fast bacterium that can cause a range of infections, including pneumonia and septicemia.
**Option D:** Isospora is a protozoan parasite that exhibits acid-fastness due to the presence of a mycolic acid-rich cell wall. It is a coccidian parasite that can cause intestinal infections in immunocompromised individuals.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that acid-fastness is not specific to mycobacteria and can be exhibited by other microorganisms, including actinomycetes and protozoa. This property can be useful in diagnosing infections caused by these organisms, but it must be interpreted in the context of other clinical and laboratory findings.

**✓ Correct Answer: B. Fusobacterium nucleatum**