**Core Concept**
The underlying principle in this scenario is the increased susceptibility of immunocompromised patients to opportunistic infections, particularly those caused by fungi and bacteria. Neutropenia, a condition characterized by an abnormally low count of neutrophils (a type of white blood cell), compromises the body's defense against these pathogens.

**Why the Correct Answer is Right**
In immunocompromised patients, such as those who have undergone transplantation, the risk of developing fungal infections is significantly higher. **Candida** species, particularly **Candida albicans**, are common opportunistic pathogens in these patients. The presence of pyrexia (fever) and neutropenia in this patient suggests a possible fungal infection, with **Candidemia** being a likely diagnosis. Candidemia occurs when **Candida** enters the bloodstream, which can happen when a patient's skin or mucous membranes are compromised, allowing the fungus to invade the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because viral infections, while possible in immunocompromised patients, are less likely to cause neutropenia as a primary symptom. Additionally, viral infections often present with different clinical manifestations, such as rash or gastrointestinal symptoms.
**Option B:** This option is incorrect because **Pseudomonas aeruginosa** is a bacterium that can cause infections in immunocompromised patients, but it is not the most likely cause of pyrexia and neutropenia in this scenario. **Pseudomonas** infections often present with different symptoms, such as pneumonia or urinary tract infections.
**Option C:** This option is incorrect because **Toxoplasma gondii** is a protozoan parasite that can cause infections in immunocompromised patients, particularly those with HIV/AIDS. However, **Toxoplasma** infections are less likely to cause neutropenia as a primary symptom.

**Clinical Pearl / High-Yield Fact**
In immunocompromised patients, particularly those who have undergone transplantation, it is essential to be aware of the increased risk of fungal infections, such as candidemia. Early recognition and treatment of these infections are critical to prevent severe complications and improve patient outcomes.

**Correct Answer:** C. **Candidemia**