**Core Concept**
Primary amyloidosis, also known as AL amyloidosis, is a plasma cell dyscrasia characterized by the deposition of amyloid fibrils composed of immunoglobulin light chains (AL) in various organs, leading to organ dysfunction. The disease is often associated with a monoclonal gammopathy of undetermined significance (MGUS) or multiple myeloma.

**Why the Correct Answer is Right**
The most common cause of death in primary amyloidosis is cardiac involvement, specifically restrictive cardiomyopathy due to amyloid deposition in the heart. This leads to diastolic dysfunction, impaired cardiac filling, and ultimately, heart failure. The amyloid deposits disrupt the normal functioning of cardiac myocytes, leading to a restrictive pattern of cardiac filling and reduced cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Renal failure is a common complication of primary amyloidosis, but it is not the most common cause of death. Renal involvement can lead to nephrotic syndrome, but patients often die from cardiac complications before renal failure becomes severe.
**Option B:** Hepatic involvement can occur in primary amyloidosis, but it is not typically the most common cause of death. Liver dysfunction may lead to ascites, jaundice, and coagulopathy, but it is usually manageable with supportive care.
**Option C:** Peripheral neuropathy is a common symptom of primary amyloidosis, but it is not the most common cause of death. Neuropathy can be disabling and painful, but it is often managed with supportive care and pain management.

**Clinical Pearl / High-Yield Fact**
Amyloidosis should be considered in the differential diagnosis of any patient with a monoclonal gammopathy, particularly if they have symptoms or signs of organ dysfunction. A serum protein electrophoresis (SPEP) and immunofixation electrophoresis (IFE) can help identify a monoclonal spike, and a biopsy of affected tissue can confirm the presence of amyloid deposits.

**Correct Answer: C. Cardiac involvement. Cardiac involvement is the most common cause of death in primary amyloidosis.**