**Core Concept**
The presence of spherocytes in a peripheral blood smear suggests a hemolytic anemia, specifically a disorder of red blood cell membrane. Spherocytes are small, round erythrocytes with a reduced surface area-to-volume ratio, often resulting from the loss of membrane surface area due to mechanical stress or enzymatic degradation.

**Why the Correct Answer is Right**
The most relevant investigation for a patient with spherocytes is the direct Coombs test (also known as the direct antiglobulin test, DAT). This test detects the presence of antibodies or complement proteins attached to the red blood cell membrane, which can cause hemolysis and lead to the formation of spherocytes. The DAT is a critical diagnostic tool for identifying autoimmune hemolytic anemias, such as warm autoimmune hemolytic anemia (WAHA) or cold agglutinin disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemoglobin electrophoresis is useful for diagnosing hemoglobinopathies (e.g., sickle cell disease, thalassemia) but is not directly relevant to spherocytes.

**Option B:** A reticulocyte count can provide information on the rate of red blood cell production and destruction but does not directly diagnose the underlying cause of spherocytes.

**Option C:** A bone marrow biopsy may be useful in diagnosing various hematological disorders but is not the most relevant investigation for a patient with spherocytes.

**Clinical Pearl / High-Yield Fact**
When interpreting a peripheral blood smear, remember that spherocytes can also be seen in hereditary spherocytosis, a genetic disorder affecting the red blood cell membrane. However, the presence of family history, physical examination findings, and laboratory results (e.g., osmotic fragility test) can help differentiate between autoimmune and hereditary causes.

**Correct Answer:** C. Direct Coombs test (DAT)