**Core Concept**
Amyloidosis is a group of diseases characterized by the deposition of insoluble amyloid fibrils in various tissues, leading to organ dysfunction. Primary AL Amyloidosis is a type of amyloidosis caused by the deposition of light chain immunoglobulins produced by clonal plasma cells.

**Why the Correct Answer is Right**
In primary AL Amyloidosis, the amyloid fibrils deposit in the heart, leading to restrictive cardiomyopathy, which ultimately results in cardiac failure. The deposition of amyloid in the heart muscle impairs its ability to contract and relax, leading to decreased cardiac output and ultimately, cardiac failure. The heart is one of the most commonly affected organs in primary AL Amyloidosis, and cardiac failure is the leading cause of death in these patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Respiratory failure is not the most common cause of death in primary AL Amyloidosis, although it can occur due to the deposition of amyloid in the lungs.
**Option C:** Renal failure can occur in amyloidosis, but it is not the most common cause of death in primary AL Amyloidosis. The kidneys are affected in amyloidosis, but the heart is more commonly involved.
**Option D:** Septicemia is not directly related to the deposition of amyloid in primary AL Amyloidosis. While patients with amyloidosis may be more susceptible to infections, septicemia is not the most common cause of death in these patients.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the heart is the most commonly affected organ in primary AL Amyloidosis, and cardiac failure is the leading cause of death in these patients. This highlights the importance of cardiac evaluation and monitoring in patients with primary AL Amyloidosis.

**✓ Correct Answer: B. Cardiac failure**