PENK and acute kidney injury: Apart from the gold standard AKI functional marker, se-creatinine, several other markers have been investigated, including AKI stress markers (e.g., TIMP-2 [tissue inhibitor of metalloproteinase-2], IGFBP7 [insulin-like growth factor binding protein]), damage markers (e.g., NGAL, KIM-1 [kidney injury molecule 1]) and functional markers (e.g., Cystatin C, penKID [proenkephalin A 119–159]) with various success [8,10].