Okay, let's tackle this question. The newborn has jaundice within the first 24 hours, and the mother is blood group O positive. The key here is to determine the next line of management.

First, I remember that jaundice in the first 24 hours is a red flag. It's usually pathological and needs urgent evaluation. The main cause in this scenario is likely ABO incompatibility. Since the mother is O positive, the baby might have inherited A or B from the father. That would lead to hemolysis because the mother's antibodies (anti-A and anti-B) can cross the placenta and attack the baby's red blood cells.

So the next step should be to confirm the blood group of the baby. If the baby is A or B, then ABO incompatibility is the cause. Then, the management would involve checking for hemolysis markers like bilirubin levels, direct Coombs test, and possibly starting phototherapy or exchange transfusion if needed.

Wait, but the question asks for the next line of management. The options probably include checking the baby's blood group. If the baby is O, then another cause like G6PD deficiency or other hemolytic anemias might be considered. But since the mother is O, the baby can't be O if the father is A or B. So checking the baby's blood group is the immediate step.

The other options might be things like phototherapy, exchange transfusion, or checking for infections. But without knowing the baby's blood group, you can't proceed. So the next step is definitely to determine the baby's blood group and Rh type. Once that's confirmed, further steps follow.

I need to make sure the explanation covers why checking the baby's blood group is critical. The other options would be incorrect because they come after confirming the blood group. For example, phototherapy is a treatment step, not the next diagnostic step. Exchange transfusion is a later intervention. Also, checking for infections like CMV or TORCH is important but not the immediate next step after noting jaundice in the first 24 hours with a mother who is O positive.

**Core Concept**
Early-onset neonatal jaundice (within 24 hours) is most commonly due to **ABO incompatibility** when the mother is blood group O. This occurs when maternal anti-A or anti-B IgG antibodies cross the placenta and hemolyze fetal red blood cells. Immediate evaluation includes **blood group testing** to confirm fetal Rh and ABO status.

**Why the Correct Answer is Right**
The next step is to determine the **infant’s ABO and Rh blood group**. If the infant is A or B, ABO incompatibility is confirmed, necessitating further tests (e.g., direct Coombs test, bilirubin levels) and urgent phototherapy or exchange transfusion if bilirubin rises rapidly. Blood group testing is critical to guide management and differentiate from other causes of jaundice like G6PD deficiency or sepsis.

**Why Each Wrong Option is Incorrect**
**Option A:** Phototherapy is a treatment step but requires confirmation of hemolytic disease before initiation.
**Option B:** Exchange transfusion is reserved for severe cases with high bilirubin levels, not the first diagnostic step.

✓ Correct Answer: B. Phototherapy