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: Medicine
The main diagnostic test for hereditary spherocytosis is: 
Select and explain. 
A. Osmotic fragility, B. Bone marrow study, C. Hemoglobin electrophoresis, D. Mutation study
A: Osmotic fragility
Answer: A: Osmotic fragility 
Explanation: The presence of osmotic fragility is the main diagnostic test for hereditary spherocytosis. Ref: Harrisons principles of internal medicine, 18th edition, Page 875.