D are connected somewhere, since they're at the same potential.
D really is grounded (neutral), then it may not be properly bonded. A short to the grounded (neutral) should trip the breaker.
Start by turning off the breaker, and making sure power is off. Then open up the outlet, and see if the two conductors are touching. If not, open up the panel and check. If everything still looks okay, trace the cable and look for other junctions and/or damage to the cable.
If I had to guess, I'd say it's a "bootleg ground" gone wrong. The cable feeding this receptacle is probably only 3 wires, but somebody wanted to install a 4 prong receptacle. To do this, they tried to connect the grounding terminal to the grounded (neutral) terminal ("bootleg ground"). Instead they accidentally connected the grounded (neutral) terminal to one of the ungrounded (hot) terminals.
If this is the case, you'll probably see the problem as soon as you pull the receptacle. Since there's probably a short bit of wire between two terminals.