Final answer:
An increase in spending growth with completely flexible prices results in an immediate increase in real GDP until the economy reaches its potential output, after which it leads to higher inflation.
Step-by-step explanation:
If prices are completely flexible, then an increase in spending growth will lead to an immediate increase in real GDP. This outcome is expected because when aggregate demand (AD) shifts to the right, it leads to a greater real GDP and to upward pressure on the price level. However, once the economy is at potential output, the Aggregate Supply (AS) curve becomes vertical. In this scenario, any further increase in AD will not affect the output (real GDP) but will lead to an increase in the price level, known as inflation, as per Keynesian economics.