Step 1: The question hinges on which agent mimics erythroblastosis fetalis, and congenital CMV is the classic culprit through its hepatosplenomegaly, jaundice and haemolytic-type blood findings in the newborn. Step 2: CMV is a herpesvirus with a broad spectrum, often silent but capable of a mononucleosis-like syndrome of fever and fatigue in older patients. Step 3: The distinction from EBV is useful - both give a mono picture, but EBV adds a marked sore throat that CMV characteristically lacks, so the resemblance to erythroblastosis still points to CMV. Step 4: HSV (vesicular lesions, neonatal disseminated disease of a different pattern) and Staphylococcus (pyogenic bacterial infection) do not reproduce the erythroblastosis-like presentation. \[\boxed{\text{CMV}}\]