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. Immunoglobulin assay, B. Ultrasound, C. Rectal biopsy, D. Urine examination
C: Rectal biopsy
Answer: C: Rectal biopsy 
Explanation: The best sites from which to obtain a biopsy specimen in systemic amyloidosis are the abdominal fat pad and rectal mucosa (approaching 90% sensitivity for fat pad and 73-84% for rectal mucosa)
While some imaging modalities can strongly suggest amyloidosis, a tissue sample showing birefringent material is still the criterion standard.
This is definitely the case in terms of cardiac-specific amyloid.
While endomyocardial biopsy is the best confirmatory > test for local cardiac amyloid deposition, it can be very- risk averse and requires a centre of excellence with the full complement of immunohistochemical and molecular-based testing