Leptospirosis: This diagnosis is highly plausible given the symptoms of jaundice, acute kidney injury (AKI), leukocytosis, and fever, especially in a returning traveler from Costa Rica, where leptospirosis is endemic. The disease is caused by the bacterium Leptospira, often contracted through water or soil contaminated with the urine of infected animals.

Dengue Fever: Although dengue fever primarily presents with fever, headache, and thrombocytopenia, severe cases can lead to hemorrhagic manifestations and organ impairment, including liver dysfunction and AKI. Costa Rica is an area where dengue fever is prevalent.
Malaria: While the primary symptoms of malaria include fever, chills, and flu-like symptoms, severe malaria can cause jaundice due to hemolysis and AKI. Given the travel history, malaria should be considered, especially if the traveler did not adhere to prophylactic measures.
Viral Hepatitis: Hepatitis A and E are considerations in travelers to endemic areas, and both can cause jaundice. However, the presence of AKI and leukocytosis might suggest a more severe or complicated infection.

Hantavirus: Although less common, hantavirus infection can lead to hantavirus pulmonary syndrome (HPS) or hemorrhagic fever with renal syndrome (HFRS), both of which can present with fever, renal failure, and in severe cases, jaundice. The risk is lower but the severity of the disease warrants consideration.
Rickettsial Diseases: Diseases like Rocky Mountain spotted fever or typhus can present with fever, rash, and in severe cases, organ dysfunction including AKI. These are less likely but critical to diagnose early due to the need for specific antibiotic treatment.
Severe Bacterial Sepsis: Any bacterial infection leading to sepsis can cause the combination of symptoms described, including jaundice due to liver dysfunction, AKI, leukocytosis, and fever. Early recognition and treatment of sepsis are crucial.

Yellow Fever: Although vaccination is usually required for travel to certain areas, if the traveler was not vaccinated or the vaccine failed, yellow fever could be a consideration. It presents with fever, jaundice, and can lead to AKI and hemorrhagic manifestations.
Toxoplasmosis: Reactivation of toxoplasmosis in an immunocompromised host could potentially lead to a wide range of symptoms including fever and organ dysfunction, but it would be an unusual cause of this specific constellation of symptoms in a returning traveler.
Chikungunya: While primarily known for causing fever and arthralgia, chikungunya virus infection can rarely lead to more severe manifestations, including neurological and renal complications. However, jaundice and AKI would be uncommon presentations.