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
In which of the following rheumatoid factor is not present in serum? 
Select and explain. 
A. Rheumatoid arthritis, B. Felty's syndrome, C. Still's disease, D. Present in all of the above
C: Still's disease
Answer: C: Still's disease 
Explanation: Still's disease is a variant of rheumatoid arthritis where rheumatoid factor is not seen.