A 3 months infant has presented with recurrent infections. He has abnormal facies and CXR shows absent thymic shadow. What is the most likely diagnosis?

The key is C. DiGeorge syndrome. [Points in favour: i) Early age of onset ii) abnormal facies iii) absent thymic shadow on Chest X-ray iii) history of recurrent infection [Infections are common in children due to problems with the immune system's T-cell-mediated response that in some patients is due to an absent or hypoplastic thymus]. In newborn can be recognized by convulsions from hypocalcaemia due to malfunctioning parathyroid glands and low level of parathyroid hormones].