A 4-year-old male presenting with frequent falls, clumsily rising from seated positions, but a normal creatine kinase.

A 4-year-9-month-old male presented with progressive gait instability, frequent falls, and mild impaired coordination, initially raising concern for a primary neuromuscular disorder. Duchenne muscular dystrophy (DMD) was considered because of the patient's age, sex, and motor weakness. However, serum creatine kinase was not elevated, and initial DMD molecular testing was negative. Further evaluation identified truncal ataxia, oculomotor apraxia, recurrent respiratory infections, lymphocytopenia, and markedly elevated alpha-fetoprotein (AFP). These findings guided the differential toward a disorder related to DNA-damage. Targeted molecular testing identified compound heterozygous pathogenic variants in ATM, establishing a diagnosis of ataxia-telangiectasia (A-T). This case illustrates how phenotype-directed laboratory testing can elucidate diagnostic probability, prevent an inaccurate initial diagnosis, and guide precision molecular testing. Early recognition of A-T permits coordinated neurologic, immunologic, malignancy-surveillance, and genetic-counseling interventions.