**Core Concept**
The patient's presentation of anemia, mild jaundice, and spherocytes on peripheral smear is suggestive of a hemolytic disorder, specifically a condition where red blood cells are destroyed prematurely. This condition is often due to an inherited or acquired defect in the red blood cell membrane.

**Why the Correct Answer is Right**
The patient's symptoms and lab findings are consistent with Hereditary Spherocytosis (HS), a genetic disorder characterized by the production of abnormal red blood cells that are more prone to hemolysis. The spherocytes are created due to mutations in genes encoding proteins of the red blood cell membrane, such as spectrin, ankyrin, band 3, or protein 4.2. The diagnosis of HS can be confirmed by demonstrating the presence of these mutations or by performing a **Reticulocyte count**, which would be elevated in this condition.

**Why Each Wrong Option is Incorrect**
* **Option A:** While a Coombs test can be used to diagnose autoimmune hemolytic anemia, it would not be the best investigation for this patient, as the presence of spherocytes on the peripheral smear is more consistent with a congenital condition like HS.
* **Option B:** A bone marrow biopsy may be useful in diagnosing other conditions affecting the bone marrow, such as aplastic anemia, but it is not the best investigation for this patient, as the clinical presentation and lab findings are more suggestive of a hemolytic disorder.
* **Option C:** A liver function test may be useful in evaluating the patient's jaundice, but it would not be the best investigation for diagnosing the underlying cause of the patient's anemia and jaundice.

**Clinical Pearl / High-Yield Fact**
Hereditary Spherocytosis is an autosomal dominant disorder, meaning that if one parent is affected, there is a 50% chance that each child will inherit the mutated gene.

**Correct Answer:** C. Reticulocyte count.