Differential Diagnosis for 28-month-old Female with Intermittent Fever and Leukocytosis

Viral Infection: Given the age of the patient and the presentation of intermittent fever with a significant leukocytosis (46,000 leukocytes) and neutrophilia, a viral infection is a plausible cause. Many viral infections can cause such a presentation, and the body's response to viral infections often includes an increase in white blood cell count.

Bacterial Infection: Although the presence of neutrophilia can suggest a bacterial infection, the intermittent nature of the fever and the high white blood cell count could also be seen in certain bacterial infections, especially those that are partially treated or are causing a low-grade, chronic infection.
Urinary Tract Infection (UTI): UTIs are common in young children and can present with fever and leukocytosis. The neutrophil predominance supports this diagnosis, as UTIs typically cause a neutrophilic response.
Pneumonia: Community-acquired pneumonia could present with fever and an elevated white blood cell count with neutrophilia, especially if caused by bacterial pathogens.

Meningitis: Although less likely given the absence of specific neurological symptoms, meningitis (especially bacterial) is a critical diagnosis not to miss due to its high morbidity and mortality. It can present with fever and elevated white blood cell count.
Septicemia: Bacterial sepsis is a life-threatening condition that requires immediate recognition and treatment. The presence of fever and significant leukocytosis with neutrophilia warrants consideration of sepsis, especially if the patient shows signs of systemic illness.
Osteomyelitis or Septic Arthritis: These conditions can present with fever and elevated white blood cell count. They are important to consider, especially if there are focal signs of infection or bone and joint involvement.

Leukemia: Although rare, leukemia can present with fever and an elevated white blood cell count. The presence of neutrophilia might be less typical for some forms of leukemia, but it's crucial to consider in the differential diagnosis due to its severity and the need for prompt treatment.
Juvenile Idiopathic Arthritis (JIA): Systemic JIA can present with fever, rash, and elevated white blood cell count, including neutrophilia. It's a diagnosis of exclusion but should be considered in a child with prolonged fever and systemic symptoms without a clear infectious cause.
Kawasaki Disease: This condition can present with fever, rash, and elevated white blood cell count. Although it typically includes other specific criteria (such as conjunctivitis, changes to the lips and oral cavity, and cervical lymphadenopathy), it's a rare but important diagnosis to consider due to its potential for cardiac complications.