A syncopal episode occurring immediately upon standing is most consistent with orthostatic hypotension, which results from a sudden drop in blood pressure due to inadequate autonomic vasoconstriction. When a person stands, gravity causes blood to pool in the lower extremities, decreasing venous return and cardiac output. In healthy individuals, compensatory mechanisms rapidly restore blood pressure. However, failure of these mechanisms leads to cerebral hypoperfusion and syncope. This contrasts with vasovagal syncope, which usually has a prodrome and occurs after standing for some time. Therefore, the immediate onset upon standing strongly indicates orthostatic hypotension.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.