Disseminated Intravascular Coagulation (DIC): Given the patient's serially dropping platelet count, high WBC count, and worsening renal function, DIC is a strong consideration. The presence of a glioblastoma multiforme could be a trigger for DIC, and the patient's clinical presentation is consistent with this diagnosis.

Sepsis: Although the patient is on meropenem and cultures are negative, sepsis could still be a contributing factor to thrombocytopenia and renal dysfunction. The high WBC count and elevated urea and creatinine levels support this possibility.
Thrombotic Thrombocytopenic Purpura (TTP): This condition is characterized by thrombocytopenia, microangiopathic hemolytic anemia, renal failure, and neurological symptoms. The patient's altered sensorium and thrombocytopenia could be indicative of TTP.
Hemophagocytic Lymphohistiocytosis (HLH): This rare condition is characterized by excessive immune activation, leading to thrombocytopenia, renal failure, and other systemic symptoms. The patient's high WBC count and thrombocytopenia could be consistent with HLH.

Heparin-Induced Thrombocytopenia (HIT): Although not explicitly mentioned, if the patient has been exposed to heparin, HIT is a critical diagnosis to consider, as it can lead to severe thrombocytopenia and thrombosis.
Drug-Induced Thrombocytopenia: Certain medications, including antibiotics like meropenem, can cause thrombocytopenia. It is essential to review the patient's medication list to rule out this possibility.

Paroxysmal Nocturnal Hemoglobinuria (PNH): This rare condition is characterized by hemolytic anemia, thrombocytopenia, and renal failure. Although unlikely, PNH could be considered in the differential diagnosis.
Atypical Hemolytic Uremic Syndrome (aHUS): This rare condition is characterized by thrombocytopenia, microangiopathic hemolytic anemia, and renal failure. The patient's clinical presentation could be consistent with aHUS, although it is a rare diagnosis.