**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, leading to toxin-mediated damage and various systemic complications. The toxins released by the bacteria cause local tissue damage, myocarditis, and neuropathy.

**Why the Correct Answer is Right**
The correct answer is related to the cardiac complications of diphtheria. The toxin released by Corynebacterium diphtheriae causes cardiac damage by inhibiting protein synthesis, leading to myocarditis, which can progress to heart failure and arrhythmias. This is due to the toxin's action on the host's ribosomes, preventing the synthesis of essential proteins.

**Why Each Wrong Option is Incorrect**
**Option A:** Myasthenia gravis is an autoimmune disease that affects the neuromuscular junction, not typically associated with diphtheria.
**Option B:** Polyneuritis, or Guillain-Barré syndrome, is a complication of diphtheria, characterized by demyelination of peripheral nerves.
**Option C:** Renal failure is not a direct complication of diphtheria, although it can occur due to myocardial infarction or heart failure secondary to diphtheria.

**Clinical Pearl / High-Yield Fact**
Diphtheria toxin causes local tissue damage and systemic complications, including myocarditis, neuropathy, and respiratory failure. Early treatment with antitoxin and antibiotics is crucial to prevent long-term sequelae.

**Correct Answer: B. Polyneuritis**