**Core Concept**
The girl's symptoms of progressive pallor and hepatosplenomegaly since birth suggest a possible diagnosis of beta-thalassemia major, a severe form of thalassemia characterized by significant anemia and bone marrow expansion leading to organomegaly. Beta-thalassemia major is caused by mutations in the HBB gene encoding the beta-globin subunit of hemoglobin.

**Why the Correct Answer is Right**
The most relevant test for diagnosing beta-thalassemia major is a hemoglobin electrophoresis, which separates and identifies different hemoglobin variants in the blood. This test is crucial for identifying the presence of beta-globin chain disease, which is characteristic of beta-thalassemia major. The test can also differentiate between beta-thalassemia major and other forms of anemia, such as alpha-thalassemia or sickle cell disease.

**Why Each Wrong Option is Incorrect**
**Option A:** A complete blood count (CBC) may show anemia, but it is not specific for diagnosing beta-thalassemia major.
**Option B:** A liver function test (LFT) may show elevated liver enzymes due to hepatosplenomegaly, but it is not diagnostic for beta-thalassemia major.
**Option C:** A bone marrow biopsy may show marrow expansion and erythroid hyperplasia, but it is not specific for diagnosing beta-thalassemia major and may not be necessary for diagnosis.

**Clinical Pearl / High-Yield Fact**
Beta-thalassemia major is a genetic disorder that requires early diagnosis and management to prevent complications such as heart failure, osteoporosis, and organ damage. Regular hemoglobin electrophoresis is essential for monitoring the effectiveness of treatment and adjusting management plans as needed.

**Correct Answer:** C. Bone marrow biopsy is not the most relevant test for diagnosing beta-thalassemia major.