B  

The patient is awake but has transient retrograde amnesia and a single episode of vomiting after minor head trauma—classic features of a mild traumatic brain injury (concussion). Diffuse axonal injury usually produces immediate prolonged coma, drunkenness alone does not cause focal post-traumatic amnesia, and cerebral venous thrombosis presents differently; therefore concussion is the most likely explanation [Pälvimäki, 2011, PMID 22204145; Collins et al., 2003, PMID 12855924].