What is most likely the cause of light-headedness and dizziness experienced by an older adult who moves from a lying to a standing position?

When moving from supine to standing, gravity causes blood to pool in the lower limbs, decreasing venous return to the heart and reducing preload. This lowers stroke volume and cardiac output, dropping systemic blood pressure and causing transient cerebral hypoperfusion - a response that is exaggerated in older adults due to diminished baroreceptor sensitivity and vascular compliance.

Blood pressure falls rather than rises during the positional change, and it is the resultant hypotension - not hypertension - that causes the symptoms.

Stroke volume decreases as a direct consequence of reduced venous return and preload, not increases.

Heart rate reflexively increases via the baroreceptor reflex as a compensatory response to the drop in blood pressure; it does not decrease.