**Core Concept**
Aspergillus species are a common cause of invasive aspergillosis, a serious and potentially life-threatening fungal infection, particularly in immunocompromised individuals. This concern is heightened in the context of a bone marrow transplant unit (BMTU) where patients are at increased risk of infection due to compromised immune systems.

**Why the Correct Answer is Right**
Aspergillus species, particularly Aspergillus fumigatus, are ubiquitous in the environment and can be found in soil, water, and decaying organic matter. When walls and floors are removed, spores can be released into the air, posing a significant risk of infection to patients and staff. The frequency and lethality of Aspergillus infections are a major concern in BMTUs, as they can manifest as invasive aspergillosis, which has a high mortality rate. Aspergillus species can infect the lungs, brain, and other organs, and the infection can spread rapidly if not promptly treated.

**Why Each Wrong Option is Incorrect**
**Option A:** Histoplasma capsulatum is a concern in areas with high bird populations, particularly in the Ohio and Mississippi River Valleys. While it can cause histoplasmosis, a lung infection, it is not as commonly associated with BMTUs as Aspergillus.

**Option B:** Blastomyces dermatitidis is a cause of blastomycosis, a fungal infection that typically affects the lungs, skin, and bones. It is more commonly seen in immunocompromised individuals, but it is not as frequent or lethal as Aspergillus infections in BMTUs.

**Option C:** Coccidioides immitis is a cause of coccidioidomycosis, a fungal infection that can affect the lungs and other organs. It is more commonly seen in the southwestern United States and is not as frequently associated with BMTUs as Aspergillus.

**Clinical Pearl / High-Yield Fact**
Aspergillus species are often referred to as "opportunistic pathogens" because they take advantage of compromised immune systems to cause infection. This is a key concept to remember when assessing the risk of fungal infections in BMTUs.

**Correct Answer:** C. Aspergillus. Aspergillus species are the biggest concern in BMTUs due to their frequency and lethality.