A 23-year-old female presented with jaundice and pallor for 2 months. Her peripheral blood smear shows the presence of spherocytes. The most relevant investigation to arrive at a diagnosis is:
The presence of spherocytes can be seen in the following conditions:

Hereditary spherocytosis
Autoimmune hemlolytic anemia
G6PD deficiency
Infections
Burns
Hemolytic disease of new born
PNH is not a cause for the presence of spherocytes; so, no test for this condition is required.
Osmotic fragility is increased with spherocytes. So, it does not add anything to our existing information about the disease causing spherocyte formation.
Reticulocyte count is expected to be elevated in the setting of haemolytic anemia (suggested by jaundice and pallor).

Coombs test is done for detection of the antibodies formed against the normal antigens present on the surface of cells like platelets and red blood cells. So, it is going to be positive in autoimmune hemolytic anemia whereas negative in hereditary spherocytosis. Thus, a differentiation between these conditions can be done.
Tests for PNH
Osmotic fragility test
Coombs test
Reticulocyte count