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Pentasomy X syndrome is a very rare sex chromosome numerical anomaly of unknown frequency. The karyotype consists of 49,XXXXX. Musculoskeletal, craniofacial, cardiac, and kidney anomalies accompany psychomotor developmental delays. This report describes, a 16-month-old girl who presented to the pediatric neurology outpatient clinic with complaints of joint laxity and inability to hold her head upright from the age of 3-4 months. The patient exhibited dysmorphic facial features and hand-foot deformities. Genetic consultation was requested, and cytogenetic examination revealed a 49,XXXXX chromosomal anomaly. The most prominent clinical feature of 49,XXXXX patients with pentasomy is severe hypotonia. This article emphasizes the importance of cytogenetic analysis in the evaluation of hypotonicity.