Tissue Penetration of Antibiotics in CriTically Ill Children

This clinical study evaluates the extent to which three commonly administered beta-lactam antibiotics (piperacillin-tazobactam, meropenem, and amoxicillin-clavulanic acid) penetrate tissue in critically ill pediatric patients. The primary objectives of this study are to: * Determine whether tissue penetration of beta-lactam antibiotics is impaired in critically ill pediatric patients during initial dosing and steady-state conditions. * Evaluate whether antibiotic concentrations within interstitial tissue fluid achieve defined pharmacokinetic and pharmacodynamic targets. * Assess the impact of patient demographics and clinical characteristics on antibiotic disposition in muscle tissue. Investigators will measure unbound antibiotic concentrations in tissue fluid using microdialysis, a technique involving the placement of a microdialysis catheter into the muscle tissue. Participating patients will undergo the following procedures: * Insertion of a flexible microdialysis catheter into the vastus lateralis (thigh) muscle under continuous sedation. * Administration of the prescribed antibiotic therapy in accordance with standard medical care. * Collection of microdialysate and blood samples across specified dosing intervals to quantify tissue and plasma drug concentrations.