**Core Concept**
Severe anemia with congestive heart failure in a young child requires immediate correction of the anemia to improve cardiac output and reduce the workload on the heart. Packed red blood cell transfusion is a crucial step in managing acute anemia, especially when the child is symptomatic.

**Why the Correct Answer is Right**
Packed red blood cell transfusion is the most appropriate immediate therapy for this boy because it rapidly increases the red blood cell mass, thereby improving oxygen delivery to tissues and reducing the severity of anemia. This is essential in a child with congestive heart failure, as the heart is already under significant stress. The packed red blood cells are concentrated, which allows for a larger volume of red blood cells to be transfused in a smaller volume of fluid, reducing the risk of transfusion-related complications. This is particularly important in a child with congestive heart failure, where fluid overload can exacerbate the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Parenteral iron injection is not an immediate therapy for severe anemia, as it takes several days to weeks to increase red blood cell production. It is typically used as a long-term solution to manage iron deficiency anemia.
**Option B:** Partial exchange transfusion is a procedure used to manage certain types of anemia, such as sickle cell disease or hemolytic disease of the newborn. It is not the most appropriate immediate therapy for this boy with severe anemia and congestive heart failure.
**Option C:** Whole blood transfusion is not necessary in this scenario, as the child only requires red blood cells to be transfused. Whole blood transfusion carries a higher risk of transfusion-related complications, such as fluid overload and coagulopathy.

**Clinical Pearl / High-Yield Fact**
In pediatric patients with severe anemia and congestive heart failure, packed red blood cell transfusion should be initiated promptly to improve cardiac output and reduce the workload on the heart. This is a critical step in managing acute anemia and can be life-saving in severe cases.

**✓ Correct Answer: A. Packed cell transfusion**