This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
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 - 
Select and explain. 
A. Reticulocyte count, B. Osmotic fragility test, C. Coombs test, D. Tests for PNH
C: Coombs test
Answer: C: Coombs test 
Explanation: Young female, with 2 years history of anemia and peripheral blood demonstrating spherocytes points towards the diagnosis of auto-immune hemolytic anemia.
Immune hemolytic (autoimmune hemolytic) anemia is the most common cause of spherocytes.
It is caused by auto antibodies against RBCs.
Detection of autoantibodies by Coombs test will establish the diagnosis.

About option b

Spherocytes are also seen in hereditary spherocytosis, in which diagnosis is established by osmotic fragility test.
However, hereditary spherocytosis is usually presents in childhood or infancy.
Whereas as autoimmune hemolytic anemia is common in young females and it is the most common cause of spherocytes in the blood.