**Core Concept**
Amyloid disease, also known as amyloidosis, is a group of conditions characterized by the deposition of amyloid fibrils in various tissues and organs. These fibrils are composed of misfolded proteins that accumulate and disrupt normal tissue architecture and function.

**Why the Correct Answer is Right**
The most reliable investigation in amyloid disease is biopsy, specifically a bone marrow biopsy. This is because amyloid fibrils can be directly visualized in the biopsy specimen, allowing for a definitive diagnosis. Immunohistochemical staining can also be used to identify the specific type of amyloid protein involved. This method is considered the gold standard for diagnosing amyloidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests, such as serum protein electrophoresis (SPEP) or urine protein electrophoresis (UPEP), can detect abnormal proteins in the blood or urine, but they are not specific for amyloid disease and can be false-positive or false-negative.
**Option B:** Imaging studies, such as chest X-rays or CT scans, can show organ involvement, but they cannot directly detect amyloid fibrils and are not specific for amyloid disease.
**Option C:** Echocardiography can show cardiac involvement, but it is not a reliable method for diagnosing amyloidosis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that biopsy is the only way to definitively diagnose amyloidosis, and the type of biopsy (e.g., bone marrow, abdominal fat, or rectal biopsy) depends on the suspected location of amyloid deposition.

**Correct Answer: C. Echocardiography can show cardiac involvement, but it is not a reliable method for diagnosing amyloidosis.