Tag applied, ball dropped - Obstruction called

[Dakota]

Let's see...

1. Catcher sets up blocking the plate without the ball.
2. Catcher catches the thrown ball before the runner arrives. Possession of the ball now makes the block legal.
3. Runner slides into catcher's foot, the slide stops short of the plate but she knocks the ball loose. Slide makes the contact with the catcher legal (assuming legal slide).
4. Catcher reaches for the ball (no step required) picks up ball and tags runner.

Sounds like an out to me. ASA - too lazy to look up NSA at the moment.