Concurrent combined methylmalonic acidemia and homocystinuria with down syndrome in a Chinese preschool Child: An in-depth case report and literature review.

Background: Dual occurrence of distinct genetic diseases is exceptionally rare, complicating both diagnosis and management when the conditions share overlapping symptoms. Case presentation: We describe a preschooler girl diagnosed with Down syndrome at 27 months who developed unexplained motor regression with age. Extensive investigations were carried out to elucidate the etiology, encompassing comprehensive neuromuscular and skeletal assessments, radiographic evaluations of the joints, electrophysiological studies, cerebral-spinal magnetic resonance imaging (MRI), hematological biochemical assays, plasma ammonia and lactate levels, full blood count analyses, echocardiography, and chromatography-mass spectrometry-based testing of amino acids, fatty acids, and organic acid metabolites in both blood and urine. Notably, significantly elevated levels of homocysteine and propionylcarnitine were detected in her blood, while urinary methylmalonic acid was also found to be abnormally high. Trio-whole exome sequencing confirmed the diagnosis as Combined methylmalonic acidemia and homocystinuria (Combined MMA and HCU), specifically due to a cblC defect, resulting from two compound heterozygous pathogenic mutations (c.217C > T and c.482G > A) in the Conclusion: Our study highlights the special and intriguing aspects of managing Combined MMA and HCU, emphasizing the value of a comprehensive diagnostic approach that integrates clinical acumen, metabolic screening, and sophisticated molecular analyses for achieving precise diagnoses in such intricate cases.