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
The most reliable investigation in amyloid disease is - 
Select and explain. 
A. Rectal biopsy, B. Immunoglobulin assay, C. Ultrasound, D. Urine examination
A: Rectal biopsy
Answer: A: Rectal biopsy 
Explanation: Ans. is 'a' i.e., Rectal biopsy