## Core Concept
Amyloidosis is a group of diseases characterized by the deposition of amyloid fibrils in various tissues of the body. The diagnosis of amyloidosis involves demonstrating the presence of amyloid deposits in tissues. This can be achieved through various methods, including histopathological examination, biochemical tests, and imaging studies.

## Why the Correct Answer is Right
The correct answer, **C. Histopathological examination with Congo red staining**, is considered the gold standard for confirming amyloidosis. Histopathological examination involves analyzing tissue samples under a microscope to detect the presence of amyloid deposits. Congo red staining is a specific technique used to identify amyloid fibrils, as it binds to the fibrils and exhibits a characteristic apple-green birefringence under polarized light. This method provides a definitive diagnosis of amyloidosis and can also help identify the type of amyloid protein deposited.

## Why Each Wrong Option is Incorrect
* **Option A:** Biochemical tests, such as serum protein electrophoresis (SPEP) or urine protein electrophoresis (UPEP), can detect abnormal proteins in the blood or urine but are not definitive for diagnosing amyloidosis.
* **Option B:** Imaging studies, such as echocardiography or magnetic resonance imaging (MRI), can suggest the presence of amyloidosis by showing characteristic organ involvement (e.g., left ventricular hypertrophy in cardiac amyloidosis) but cannot confirm the diagnosis.
* **Option D:** Immunohistochemistry can help identify the type of amyloid protein deposited but is not the primary method for confirming amyloidosis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that amyloidosis can manifest in various organs, including the kidneys, heart, liver, and nerves. When suspecting amyloidosis, it is essential to consider the clinical presentation and obtain tissue samples from affected organs for histopathological examination with Congo red staining.

## Correct Answer: C. Histopathological examination with Congo red staining