Use GPIO to disable voltage divider
First thing - if the ADC is okay with 50K you can use 150K and 75.0K (the source impedance will be exactly 50K).

The current used by the divider will be 4.3V/225K = 19.1uA.

Unlike most micros, I think you can actually lift the lower end of the divider and reduce the current, if you pick a 5V-tolerant input that is shared with the ADC and use another 5V-tolerant pin for the divider control. At least that is what it looks like to me. You would set the control pin to low/output for divider operation and have the ADC input active. To disable set both pins to digital inputs.