RESUMEN
A male patient in his late 30s with a history of Lynch syndrome and colorectal cancer relapse, which recently started chemotherapy, was admitted to the emergency department with acute lower limb weakness that had progressed to all limbs and resulted in complete flaccid paresis with general areflexia. Blood tests showed severe hyperkalaemia, severe acute kidney injury and hyperuricaemia. Ultrasound showed bilateral hydronephrosis due to pelvic mass obstruction. Hyperkalaemia correction measurements were started as well as rasburicase with the assumption of tumour lysis syndrome and postrenal kidney injury. The patient showed a favourable clinical response with complete return of limb movement in the following hours and progressive recovery of renal function in the following days. This case highlights the need for prompt diagnosis and correction of severe hyperkalaemia, and its multiple possible causes, as it can lead to acute flaccid paralysis and a fatal outcome.