**Core Concept**
Amyloidosis refers to the accumulation of amyloid, a fibrillar proteinaceous material, in various tissues and organs. Diagnosing amyloidosis requires demonstrating the presence of amyloid deposits in affected tissues.

**Why the Correct Answer is Right**
A rectal biopsy is the preferred method for confirming amyloidosis due to its high sensitivity and specificity. The rectal submucosa is a common site for amyloid deposition, and a biopsy from this area can provide a definitive diagnosis. This is because the rectal submucosa contains a high concentration of amyloid-laden vessels, making it an ideal location for detecting amyloid deposits. The biopsy procedure is also relatively straightforward and minimally invasive.

**Why Each Wrong Option is Incorrect**
**Option A:** Colonoscopy is an important diagnostic tool in gastroenterology, but it is not the best method for confirming amyloidosis. While colonoscopy can provide valuable information about the colon's mucosal and submucosal layers, it is not as sensitive as a rectal biopsy for detecting amyloid deposits.
**Option B:** Sigmoidoscopy is similar to colonoscopy but has a shorter reach. It is not as effective as a rectal biopsy for confirming amyloidosis due to its limited depth of penetration and narrower field of view.
**Option D:** Tongue biopsy is sometimes used to diagnose amyloidosis, but it is not the preferred method. The tongue is a site of amyloid deposition in some cases, but a rectal biopsy is more sensitive and specific for diagnosing systemic amyloidosis.

**Clinical Pearl / High-Yield Fact**
A rectal biopsy should be performed if there is a high suspicion of amyloidosis, particularly in patients with unexplained peripheral neuropathy, nephrotic syndrome, or cardiac involvement. This procedure can provide a definitive diagnosis and guide further management.

✓ Correct Answer: C. Rectal biopsy