**Core Concept**
Primary amyloidosis, also known as light chain amyloidosis (AL amyloidosis), is a condition characterized by the deposition of insoluble fibrils composed of immunoglobulin light chains in various tissues. This deposition disrupts normal tissue structure and function, leading to a range of clinical manifestations.

**Why the Correct Answer is Right**
Cardiac involvement is the most common cause of death in primary amyloidosis. The deposition of amyloid fibrils in the heart leads to restrictive cardiomyopathy, characterized by impaired diastolic function and ultimately resulting in heart failure. The amyloid deposits disrupt the normal electrical conduction pathways in the heart, increasing the risk of arrhythmias. The cardiac involvement is often asymptomatic in the early stages, but as the disease progresses, symptoms of heart failure, such as shortness of breath and fatigue, become apparent.

**Why Each Wrong Option is Incorrect**
**Option A:** Respiratory failure is a potential complication of amyloidosis, but it is not the most common cause of death. Amyloid deposits in the lungs can lead to restrictive lung disease, but this is less common than cardiac involvement.

**Option C:** Renal failure can occur in amyloidosis due to the deposition of amyloid fibrils in the kidneys, leading to nephrotic syndrome or nephritic syndrome. However, it is not the most common cause of death in primary amyloidosis.

**Option D:** Septicemia is not a direct consequence of primary amyloidosis. While patients with amyloidosis may be more susceptible to infections due to their compromised immune system, septicemia is not the most common cause of death in this condition.

**Clinical Pearl / High-Yield Fact**
Amyloidosis can be associated with a range of underlying plasma cell dyscrasias, including multiple myeloma. In fact, up to 50% of patients with primary amyloidosis have a underlying plasma cell disorder. Therefore, a thorough evaluation for a plasma cell dyscrasia is essential in the diagnosis and management of primary amyloidosis.

✓ Correct Answer: B. Cardiac failure