A 31-year-old male farmer was admitted on December 10 with fever and low back pain for 4 days, along with facial edema and oliguria for 2 days. Physical exam: temperature 38°C, BP 130/75 mmHg, facial congestion and edema, conjunctival edema, and petechiae in the axilla. Laboratory tests: WBC 15×10<sup>9</sup>/L, platelets (PLT) 31×10<sup>9</sup>/L, atypical lymphocytes on blood smear, urinary protein (+++). Diagnosed with oliguric phase of epidemic hemorrhagic fever. The most urgent treatment for this patient is: