**Core Concept**
The patient's presentation of febrile neutropenia with worsening fever and new-onset bilateral fluffy infiltrates on chest X-ray is suggestive of an emerging complication of broad-spectrum antibiotic therapy, specifically the development of an opportunistic fungal infection.

**Why the Correct Answer is Right**
The most likely pathogen in this scenario is Pneumocystis jirovecii pneumonia (PCP), an opportunistic fungal infection that predominantly affects immunocompromised patients. The use of broad-spectrum antibiotics has altered the patient's gut flora, leading to translocation of fungi from the gastrointestinal tract to the lungs. The presence of bilateral fluffy infiltrates on chest X-ray is a classic radiographic feature of PCP.

**Why Each Wrong Option is Incorrect**
* **Option A:** Administering more broad-spectrum antibiotics would be counterproductive, as it would further disrupt the patient's gut flora and potentially exacerbate the fungal infection.
* **Option B:** Corticosteroids may be used as adjunctive therapy in PCP, but they are not the initial step in management.
* **Option D:** The patient's presentation does not suggest a viral or bacterial pneumonia, and empirical antiviral or antibacterial therapy would not be the most appropriate next step.

**Clinical Pearl / High-Yield Fact**
PCP is a common opportunistic infection in immunocompromised patients, particularly those with HIV/AIDS or undergoing chemotherapy. The diagnosis of PCP is often made radiographically, but confirmation can be achieved through induced sputum or bronchoalveolar lavage.

**Correct Answer:** B.