Which disorder has an onset in childhood or adolescence and is characterized by muscle twitches and weakness?

Myoclonic Epilepsy with Ragged Red Fibers (MERRF) is the correct answer because it is a mitochondrial disorder that typically presents in childhood or adolescence, with symptoms including myoclonic seizures, muscle weakness, and muscle twitches, specifically affecting the skeletal muscles. Additionally, individuals with MERRF may have ragged red fibers on muscle biopsy, which is a hallmark finding in the disorder. The onset and clinical manifestations align well with the characteristics described in the question.

In contrast, Neuropathy Ataxia and Retinitis Pigmentosa (NARP) primarily affects individuals due to mitochondrial dysfunction but is more associated with adult-onset symptoms and a combination of neurological deficits rather than predominantly muscular symptoms. Leigh Syndrome is a severe neurological disorder that typically manifests in infancy or early childhood, but its primary features are more focused on neurological decline and are not specifically characterized by muscle twitches and weakness as the most defining symptoms. Kearns-Sayre Syndrome presents with a triad of myopathy, ophthalmoplegia, and heart block, typically appearing in children but with more variable presentation that does not focus primarily on muscle twitching.

Therefore, MERRF stands out as the disorder that aligns closely with the symptomatology