STM32F103 - Single pin shorted to VDD?
Unsolder and carefully lift the pin of the part then measure the trace with a DMM and check for shorts to ground. If the trace is still high impedance, it's probably the MCU and the MCU will need to be replaced. Odds are the port is blown out, especially if this has been connected to a non TTL uart by mistake, or ESD damage.

While the PIN is up you can check the I/O on the PIN independent of the PCB

If the trace is shorted (still reads a value lower than 100kΩ (although should be more like few MΩ)), it's the PCB (and all PCB's are not perfect). I've had a few through hole vias that shorted planes together. The nice thing about this trace is it's only tied to one pin, which makes it easy to troubleshoot. I've had problems like this where there are 5 pins, and you have to go down the line and cut traces and remove parts until you've found the offending part or trace.