**Amyloid Disease Diagnostic Investigation**

### Core Concept
Amyloid disease, also known as amyloidosis, is a group of disorders characterized by the deposition of insoluble amyloid fibrils in various tissues, leading to organ dysfunction. The diagnosis of amyloid disease requires a reliable investigation that can detect the presence of amyloid fibrils in tissue samples.

### Why the Correct Answer is Right
The most reliable investigation in amyloid disease is biopsy with Congo red staining, which involves taking a tissue sample from a specific organ, such as the kidney or liver, and staining it with Congo red dye. This dye binds to the amyloid fibrils, causing them to exhibit a characteristic green birefringence under polarized light microscopy. This method is highly specific and sensitive for detecting amyloid deposits, making it the gold standard for diagnosing amyloid disease.

### Why Each Wrong Option is Incorrect
**Option A:** Blood tests, such as serum protein electrophoresis (SPEP) and urine protein electrophoresis (UPEP), can help identify abnormal protein production, but they are not specific for amyloid disease and require biopsy confirmation.
**Option B:** Imaging studies, such as radiolabeled serum albumin scintigraphy, can detect amyloid deposits in some organs, but they are not as sensitive or specific as biopsy with Congo red staining.
**Option C:** Histopathological examination of tissues can show amyloid deposits, but the presence of amyloid can be missed if the tissue is not properly processed or stained.

### Clinical Pearl / High-Yield Fact
Congo red staining is a simple and cost-effective method for diagnosing amyloid disease, but it requires expertise in interpreting the results. A biopsy sample should be sent to a specialized laboratory for analysis to ensure accurate diagnosis.

### Correct Answer: C. Histopathological examination of tissues can show amyloid deposits, but the presence of amyloid can be missed if the tissue is not properly processed or stained.