Introduction: Adenosine deaminase (ADA) deficiency is a rare cause of Severe Combined Immunodeficiency (SCID) and constitutes a pediatric medical emergency . Delayed recognition results in rapid clinical deterioration from overwhelming infections. However, early diagnosis is often challenging due to heterogeneous initial presentations. Case presentation: We report a 4-month-old term female born to non-consanguineous parents who presented with persistent cough from day 13 of life. Despite receiving BCG and routine immunizations, she required treatment for pneumonia on two occasions and developed a progressive generalized pigmented rash over one month. Her clinical course was urther complicated by severe failure to thrive and emerging global developmental delay . Examination revealed absence of lymphadenopathy with diffuse cutaneous hyperpigmentation. Laboratory evaluation demonstrated marked lymphopenia. Flow cytometry showed profound B, NK and T-cell deficiency , raising suspicion of SCID, which was subsequently confirmed as ADA deficiency on genetic testing. She was initiated on immunoglobulin replacement, antimicrobial prophylaxis, and multidisciplinary care with nutritional and developmental rehabilitation. Following therapy , her respiratory infections improved, and she is currently being optimized for definitive hematopoietic stem cell transplantation. Conclusion: This case underscores how subtle early clinical clues, recurrent infections, failure tothrive, and atypical dermatological manifestations, may signal an underlying lethal immunodeficiency . Heightened clinical vigilance and early immunological evaluation can transform a potentially fatal trajectory into a curable condition through timely definitive therapy .