Hemolytic Anemia: This condition involves the rapid destruction of red blood cells, which can lead to a sudden drop in hemoglobin levels. It can be caused by various factors, including autoimmune disorders, infections, or medications. The acute nature and significant hemoglobin drop are consistent with hemolytic anemia.

Sickle Cell Crisis: In patients with sickle cell disease, a crisis can occur due to various triggers, leading to rapid hemolysis and a significant drop in hemoglobin levels. The acute onset and severity of anemia make it a plausible diagnosis.
Severe Infection or Sepsis: Certain infections, especially those causing sepsis, can lead to a rapid decline in hemoglobin levels due to the body's inflammatory response and potential hemolysis. The acute nature of the condition and the potential for severe anemia align with this diagnosis.
Major Bleeding: Although not typically categorized under anemia of chronic disease, significant blood loss from any source (e.g., gastrointestinal, trauma) can lead to a rapid decrease in hemoglobin levels. The context of the question suggests looking for anemias similar to anemia of chronic disease, but major bleeding is a critical consideration in the differential diagnosis of acute anemia.

Acute Hemorrhage: Similar to major bleeding, an acute hemorrhage can cause a rapid drop in hemoglobin. It's crucial to identify and manage promptly to prevent severe outcomes.
Disseminated Intravascular Coagulation (DIC): This condition involves both clotting and bleeding and can lead to significant hemolysis and a drop in hemoglobin. It's a life-threatening condition that requires immediate attention.
Thrombotic Thrombocytopenic Purpura (TTP): A rare disorder characterized by clotting in small blood vessels throughout the body, leading to hemolytic anemia. It's critical to diagnose and treat TTP promptly due to its high mortality rate if left untreated.

Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, bone marrow failure, and the potential for blood clot formation. While rare, it could present with acute anemia.
Microangiopathic Hemolytic Anemia (MAHA): Seen in conditions like TTP and hemolytic-uremic syndrome (HUS), MAHA is characterized by the destruction of red blood cells as they pass through small blood vessels. Its inclusion in the differential is based on its potential to cause severe anemia, although it's less common than other diagnoses listed.