1) Most likely diagnosis: and why: Acute left convexity subdural hematoma. The CT shows a crescent‑shaped, hyperdense extra‑axial collection along the left cerebral convexity that crosses suture lines and abuts the hemisphere (typical for subdural), the density is consistent with acute blood, and there is marked mass effect with compression of the left lateral ventricle and a rightward midline shift of approximately 10–12 mm. Lack of a lentiform shape and no visible skull fracture on this slice make an epidural hematoma less likely. 2) Next best diagnostic step: Immediate neurosurgical consultation is the priority given the large hematoma and significant midline shift. Concurrently obtain emergent labs including coagulation profile (PT/INR, PTT), type & screen/crossmatch, and basic labs. If there is concern for active ongoing bleeding or an atypical source, consider CT angiography; otherwise do not delay definitive management for additional imaging. 3) Key differential: or confirmatory test: Key differentials include acute epidural hematoma (usually biconvex/lentiform) and acute on chronic subdural (mixed density). MRI with blood‑sensitive sequences (GRE/SWI and T1/T2) can help time the hemorrhage and detect associated cortical contusion or subacute/chronic components; CT angiography can help identify active contrast extravasation or an underlying vascular lesion if suspected. 4) Possible treatment: or management: Given the large size and >10 mm midline shift, urgent surgical evacuation (craniotomy or decompressive craniectomy / burr hole drainage depending on neurosurgical assessment) is usually indicated. Initial medical management includes airway protection/ventilation as needed, reversal of anticoagulation if present, blood product support, intracranial pressure control (head elevation, hyperosmolar therapy such as hypertonic saline or mannitol), and ICU level care with repeat imaging after intervention.