According to the skill tree, 3 points in Quicksand node on the Anomaly skill tree will let you invest points in Exacerbate. I could not put points in Exacerbate even after fully investing in Quicksand. Weirder still, I was able to place points in Void Touched, to invest in Exacerbate, and than invest in Mark of Rot, but than be able to remove a point from Exacerbate, but still not connect to Quicksand.
Edit Changed the picture to show that nodes not investing properly.