Why is this challenge room locked?

Question

I ran into this gated challenge room during a recent Binding of Isaac playthrough (with Maggie, as you can see). I had definitely killed the enemies in the room as the entrance door reopened but this one did not. I've been able to enter challenge rooms before without much fanfare. Is there something extra I have to do to open this door? Is Maggie not allowed in weapon rooms?

Answer

Challenge rooms can only be entered if the player has full health counting regular hearts and soul hearts, or if they can bypass it with the Nail item or with the Lord of the Pit item.

Since it looks like you don't have either of those items (you need to complete the game several times to get the nail and the Lord of the Pit changes your appearance), you probably don't have full health. Once you collect enough hearts or use the Yum Heart item enough to return to full health, the room should open.

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