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can appear as an alternate version of Chub. There's also a power-up called "Doctor's Remote," which is another reference to the character in question. Unsurprisingly, the ending gives you the Fetus in a Jar powerup.
Fetus from Super Meat Boy, complete with the stage title theme from that game.
One of the endings: Isaac dresses as Dr. The Mask of Infamy bears an intentional resemblance to the recurring boss Carmilla. It even has unique synergies with both BFFS! and Fire Mind, which both cause the Holy Water to leave behind a blue fire, completing the reference. Repentance buffed it to be even more like the original, since it's now thrown at enemies rather than shattering when Isaac takes damage. The Holy Water subweapon is available as a Familiar, and functions similarly to its Castlevania counterpart, leaving behind a puddle that will damage enemies. In Rebirth, the player can find Death's Touch, which turns tears into the very same sickles. One of Death's attacks is scattering spinning sickles all over the room, which fly towards the player. In Repentance, the reverse Empress card instead turns you into Magdalene, with the message "The morning sun has vanquished the horrible night." The Empress card normally transforms you into the same demon Eve becomes. When going into the One-Winged Angel form accessible through one of Eve's starting items, the transformation is denoted by the phrase "What a horrible night to have a curse". To drive the point further, you unlock it by completing the Isaac's Awakening challenge, which starts you off with the Spirit Sword, the Trinity Shield, and Mom's Bracelet, which functions similarly to the Power Bracelet. By holding down fire and then releasing, it unleashes a spin attack, and when Isaac is at full health, it throws out a Sword Beam, much like how the Master Sword typically does. Repentance adds the Spirit Sword, which gives Isaac a sword he can swing in front of himself. Afterbirth+ added a fairy that flies towards tinted rocks, entrances to hidden rooms, and rocks hiding trapdoors. Afterbirth adds a new challenge called "I Rule!", which gives you the starting items of a sword, a shield, the Ladder, and the Boomerang. Rebirth even adds a "Chub dislikes smoke!" line. Chub can be made to eat bombs to damage her, just like the Dodongo. The Stepladder from the original Zelda's functionally identical counter-part, simply called "The Ladder.". Wrath of the Lamb's candle is pulled straight from The Legend of Zelda. Teleportation items may randomly send you to a room with a guy saying " I AM ERROR.".