Final answer:
An infant with a bleeding tendency likely has vitamin K deficiency, common in newborns, especially those from multiple or premature births. Hemolytic disease of the newborn may occur in second or subsequent pregnancies when a sensitized Rh-negative mother's antibodies attack an Rh-positive fetus's red blood cells.
Step-by-step explanation:
An infant showing a tendency to bleed two days after birth is most likely suffering from a condition related to vitamin K deficiency. Newborns have low stores of vitamin K, which is crucial for blood clotting. They can also experience a rapid increase in demand for vitamin K during growth, further depleting their already low stores if not supplemented.
Babies born prematurely or from multiple pregnancies, like twins, are at an increased risk due to even poorer stores at birth. Moreover, disorders such as hemolytic disease of the newborn can lead to significant pathological blood loss, aggravating this bleeding tendency. This disease is typically a problem in the second or subsequent pregnancies because the mother's immune system becomes sensitized to the baby's red blood cells only after the first pregnancy, leading to an immune response against fetal cells in later pregnancies if there's incompatibility, such as with the Rh factor.
During birth, Rh-positive fetal erythrocytes may leak into the maternal blood after the breakage of the embryonic chorion, sensitizing the mother's immune system. If the mother is Rh-negative and the subsequent baby is Rh-positive, the mother's antibodies can cross the placenta and attack the fetal red blood cells, causing hemolysis.