Immune Thrombocytopenic Purpura (ITP): This condition is characterized by low platelet count due to immune-mediated destruction. The high mean platelet volume (MPV) can be seen in ITP as the body tries to compensate for the low platelet count by producing larger, younger platelets. The recent steroid course could be a treatment attempt for ITP, which may not have been fully effective or could be a part of the diagnostic workup.

Bone Marrow Suppression or Failure: Steroids can sometimes mask the symptoms of bone marrow failure, which can lead to a low platelet count. A high MPV might indicate that the bone marrow is trying to produce platelets but is not doing so efficiently.
Chronic Liver Disease: Liver diseases can lead to splenic sequestration and subsequent thrombocytopenia. The liver also produces thrombopoietin, a hormone that regulates platelet production, and its dysfunction could lead to changes in platelet count and size.
Viral Infections: Certain viral infections can affect the bone marrow, leading to thrombocytopenia. The high MPV could be a response to the infection as the body tries to increase platelet production.

Heparin-Induced Thrombocytopenia (HIT): Although less likely given the context, HIT is a serious condition that can occur after heparin use and can lead to both thrombocytopenia and thrombosis. The high MPV could be seen as the body responds to the platelet activation and consumption.
Thrombotic Thrombocytopenic Purpura (TTP): A rare disorder characterized by thrombocytopenia, microangiopathic hemolytic anemia, renal failure, and neurological symptoms. It's crucial to consider TTP due to its high mortality rate if left untreated.
Disseminated Intravascular Coagulation (DIC): A condition characterized by both clotting and bleeding due to the consumption of platelets and clotting factors. It can present with thrombocytopenia and could potentially have a high MPV as a compensatory mechanism.

Myeloproliferative Neoplasms (MPNs): Conditions like essential thrombocythemia can sometimes present with thrombocytopenia and a high MPV, especially if there's a component of bone marrow fibrosis or if the patient is in a spent phase.
Glycogen Storage Disease Type Ib: A rare genetic disorder that can lead to thrombocytopenia and a high MPV due to the abnormal function of the glucose-6-phosphate transporter, affecting platelet production and function.
May-Hegglin Anomaly: A rare genetic disorder characterized by thrombocytopenia, giant platelets, and leukocyte inclusions. The high MPV would be a hallmark of this condition.