RESUMO
In the pediatric emergency department, dehydrated children are one of the most frequent causes for consultation, however, the coexistence of hyponatremia with hypochloremia and metabolic alkalosis is rare. The presence of metabolic alkalosis due to chloride depletion has been reported as a form of presentation of Cystic Fibrosis (CF). OBJECTIVE: to describe a case of cystic fibrosis of unusual presen tation in a pediatric patient. CLINICAL CASE: we report a 3-month-old previously healthy male infant who presented with internal environment abnormalities consisting of metabolic alkalosis, hypona tremia, hypokalemia, and extreme hypochloremia associated with septic shock due to mixed viral- bacterial pneumonia (Rhino/enterovirus, Streptococcus pneumoniae, and Staphylococcus aureus). Cys tic fibrosis (CF) was suspected, thus the diagnosis was corroborated by sweat test and genetic study which showed the pathogenic variants c.2834C>T (p.Ser945Leu) and c.3484C>T (p.Arg1162X), both heterozygous. CONCLUSION: special attention should be paid to the existence of hypochloremia with metabolic alkalosis and hyponatremia associated or not with pulmonary disease, suspecting CF as the first option. This consideration becomes more relevant in those countries where the neonatal screening test is not widely available.