**Core Concept**
Nocardia asteroides is a gram-positive, filamentous bacterium that can cause opportunistic infections, particularly in immunocompromised individuals, such as organ transplant recipients. This bacterium exhibits acid-fastness due to its cell wall composition, which contains mycolic acids.

**Why the Correct Answer is Right**
Nocardia asteroides is a common pathogen in immunocompromised patients, including those with organ transplants. The bacterium's acid-fastness is due to the presence of mycolic acids in its cell wall, which are also found in Mycobacterium species. The modified Ziehl-Neelsen staining with 1% H2SO4 highlights this characteristic. Nocardia asteroides typically causes pulmonary infections, which can be productive and accompanied by fever, as seen in this patient.

**Why Each Wrong Option is Incorrect**
**Option A:** Actinomyces israelii is a gram-positive, filamentous bacterium, but it is not acid-fast and typically causes infections in the oral cavity or gastrointestinal tract, not in immunocompromised patients.
**Option C:** Blastomyces dermatitidis is a dimorphic fungus, not a gram-positive bacterium, and causes systemic infections, particularly in immunocompromised patients, but it does not exhibit acid-fastness.
**Option D:** Cryptosporidium parvum is a protozoan parasite that causes gastrointestinal infections, particularly in immunocompromised patients, but it is not a gram-positive bacterium and does not exhibit acid-fastness.

**Clinical Pearl / High-Yield Fact**
Nocardia asteroides infections can be treated with antibiotics, such as sulfonamides and trimethoprim-sulfamethoxazole, but the choice of antibiotic depends on the severity and site of infection. Immunocompromised patients with Nocardia infections often require prolonged treatment and close monitoring.

**✓ Correct Answer: B. Nocardia asteroides**