A Medical Retention Processing Unit (MRPU) is the previous name of the (current) Warrior Transition Unit (WTU) in the United States Army.
The MRPU was a medical unit designated for RC soldiers or US Army Reserve (USAR) who were injured or developed severe illness while on active duty. The primary duty of the MRPU was to ensure that soldiers assigned receive the proper medical care and were restored to full duty status before returning to their respective RC unit.
The MRPU has now been replaced by the WTU, or, the Warrior Transition Unit. All units or companies that were previously known as MRPU, Med Hold or Med Holdover are renamed WTU. In the case of a brigade size unit they are known at WTB.
More information about WTU is available at the Army Personnel website, or www.armyg1.army.mil under the Warrior Transition Unit Consolidated Guidance (Administrative)