Osteopetrosis is a rare group of bone disorders characterized by defective osteoclast bone resorption causing high bone mineral density. A high bone mineral density in combination with defective skeletal mineralization results in a phenotype of osteopetrorickets. We present a rare presentation of infantile osteopetrorickets in an 8-week-old female who presented with failure to thrive, hypophosphatemia, anemia, and thrombocytopenia. A skeletal survey showed increased bone density with rachitic changes. She was found to have a homozygous T-cell immune regulator 1 (TCIRG1) pathogenic mutation consistent with osteopetrosis. This highlights the importance of a clinical suspicion of osteopetrosis with this symptom constellation.