1. Presented by Caroline Early, MD and reviewed by Mark Marzinke, PhD

An otherwise healthy exclusively breastfed newborn infant born at term with appropriate weight-gain develops jaundice during hospital admission following an uncomplicated delivery. Labs reveal an elevated serum total bilirubin of 15.5 mg/dL. The patient is started on phototherapy to prevent bilirubin-induced encephalopathy ("kernicterus") while the etiology of the hyperbilirubinemia is investigated. Workup reveals the following: direct bilirubin 0.2 mg/dL (normal), hemoglobin 10.8 g/dL (low), DAT/Coombs test is negative, reticulocyte count is 5% (elevated), and microspherocytes and "bite" cells are seen on peripheral blood smear. The newborn screen is negative. The patient's blood type is O positive and the mother is O negative. This is the mother's first pregnancy, and prenatal care was at a location other than the hospital where the infant was delivered. She recalls receiving antibiotics for a UTI, but does not know whether she received Rhogam during the pregnancy.

1. What is the most likely primary cause of the patient's clinical picture?