Viral Hemorrhagic Fever (e.g., Ebola, Lassa Fever, Dengue): This diagnosis is considered the most likely due to the combination of high fever, rash, leukopenia, and thrombocytopenia, which are characteristic findings in viral hemorrhagic fevers. These diseases are known for their severe presentation and the mentioned symptoms.

Meningococcemia: This condition, caused by Neisseria meningitidis, can present with fever, rash (petechial or purpuric), and may lead to leukopenia and thrombocytopenia due to sepsis. It's a critical diagnosis to consider due to its high mortality rate if not treated promptly.
Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, RMSF presents with fever, rash, and can lead to thrombocytopenia and leukopenia. It's a tick-borne illness that requires early treatment to prevent severe outcomes.
Ehrlichiosis/Anaplasmosis: These are tick-borne diseases caused by Ehrlichia and Anaplasma species, respectively. They can present with fever, rash, leukopenia, and thrombocytopenia, making them part of the differential diagnosis for patients with these symptoms.

Septicemia due to Bacterial Infections (e.g., Staphylococcus aureus, Streptococcus pneumoniae): While not as likely to present with all these symptoms together, bacterial septicemia can lead to a severe clinical picture including fever, rash (in the case of certain bacteria like Staphylococcus aureus), leukopenia, and thrombocytopenia. Missing this diagnosis could be fatal.
Disseminated Intravascular Coagulation (DIC): This condition can result from various causes, including infections, and presents with thrombocytopenia, among other coagulation abnormalities. It's crucial to identify and treat the underlying cause to manage DIC effectively.

Thrombotic Thrombocytopenic Purpura (TTP): A rare blood disorder characterized by thrombocytopenia, microangiopathic hemolytic anemia, renal failure, and neurological symptoms. While fever and rash are not primary symptoms, they can occasionally be present, making TTP a rare consideration in this differential diagnosis.
Hantavirus Syndrome: This viral infection can cause hemorrhagic fever with renal syndrome (HFRS) or hantavirus pulmonary syndrome (HPS), both of which can present with fever, thrombocytopenia, and in some cases, rash. It's a rare but potentially deadly diagnosis that should be considered, especially in individuals with exposure to rodents or their droppings.