The organ system most affected by undernutrition in infancy and early childhood is the

Nervous system

Nervous system is made up of organized group of cells (neurons) that specialized for the conduction of electrochemical stimuli or signals from sensory receptor to other sites. Thus, these signals are sent in form of electrochemical waves that travel through thin fibers called axons. They are made up of two parts namely: Central Nervous system and Peripheral Nervous system.