Golgari Thug

Oracle Text

When Golgari Thug dies, put target creature card from your graveyard on top of your library.

Dredge 4 (If you would draw a card, instead you may put exactly four cards from the top of your library into your graveyard. If you do, return this card from your graveyard to your hand. Otherwise, draw a card.)

Card Rulings

6/7/2013 Dredge lets you replace any card draw, not just the one during your draw step.
6/7/2013 Once you decide to replace a draw using a card's dredge ability, that card can't be removed from your graveyard "in response." (Replacement effects don't use the stack.)
6/7/2013 You can't use dredge unless you're going to draw a card and the card with dredge is already in your graveyard.