Mucopolysaccharidosis type IVA (Morquio A) in twins masquerading as distal renal tubular acidosis

A twin child in early childhood presented with growth failure and multiple skeletal deformities involving both axial and appendicular skeleton. They did not have any upper limb deformity, fractures, dental anomalies, mental retardation, facial coarsening or organomegaly. The initial differentials were rickets, spondylo-epiphyseal dysplasia, related dysplastic diseases and renal tubular acidosis (RTA). Biochemical evaluations revealed mild normal anion gap metabolic acidosis with a positive urinary anion gap and prompted us to diagnose RTA based on those. However, their detailed skeletal imaging evaluations suggested dysostosis multiplex. With the suspicion of mucopolysaccharidosis (MPS) type IV, leucocyte N-acetylgalactosamine-6-sulfate sulfatase (<i>GALNS</i>) enzyme evaluation was done along with whole exome sequencing, confirming our diagnosis. The integrating clinical history, physical examination, radiology and genetic confirmation underscores the complexity of diagnosing rare disorders like MPS IVA and highlights the need for a systematic and multidisciplinary approach.