## **Core Concept**
The question describes a 6-month-old baby with severe pallor and hepatosplenomegaly, and a similar history in a sibling, suggesting a genetic or familial condition. This clinical presentation is suggestive of a hematological disorder. The investigation of choice would likely be one that can diagnose genetic or inherited conditions affecting the blood.

## **Why the Correct Answer is Right**
The correct answer, . (Bone Marrow Examination and Genetic Testing, but most specifically points towards a condition like Beta-Thalassemia which can be diagnosed with DNA analysis), is right because conditions like beta-thalassemia major (Cooley's anemia) present with severe pallor, hepatosplenomegaly due to extramedullary hematopoiesis, and a family history of similar conditions. Genetic testing can confirm the diagnosis by identifying mutations in the HBB gene responsible for beta-thalassemia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option is not specified but if it points towards a nonspecific test like a Complete Blood Count (CBC), it would not be the investigation of choice for diagnosing a specific genetic condition like beta-thalassemia, although CBC can suggest the diagnosis.
- **Option B:** - Similarly, without specifics, if this points to imaging studies like Ultrasound or CT, these would not diagnose genetic hematological disorders.
- **Option D:** - If this option suggests other tests not directly related to genetic diagnosis or specific hematological condition diagnosis, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for conditions like beta-thalassemia, which present with severe anemia and hepatosplenomegaly in infancy, along with a positive family history, genetic testing is crucial for confirmation. This guides management, including potential prenatal testing for future pregnancies.

## **Correct Answer: .**