**Core Concept**
Myasthenia gravis (MG) is an autoimmune disease characterized by the presence of antibodies against acetylcholine receptors (AChRs) at the neuromuscular junction, leading to muscle weakness and fatigue. In managing MG patients, it is essential to avoid drugs that can exacerbate neuromuscular transmission.

**Why the Correct Answer is Right**
Tubocurare is a non-depolarizing neuromuscular blocking agent that competitively inhibits the action of acetylcholine at the neuromuscular junction. In a patient with MG, the pre-existing autoantibodies against AChRs can be further exacerbated by tubocurare, potentially leading to severe muscle weakness or even respiratory failure. This is because tubocurare can bind to the remaining functional AChRs, further reducing neuromuscular transmission.

**Why Each Wrong Option is Incorrect**
* **Option A:** Succinylcholine is a depolarizing neuromuscular blocking agent that can be used cautiously in MG patients under close monitoring. It may even be beneficial in some cases to temporarily improve muscle tone.
* **Option C:** Pyridostigmine is an acetylcholinesterase inhibitor used in the management of MG to increase the concentration and duration of action of acetylcholine at the neuromuscular junction.
* **Option D:** Halothane is a volatile anesthetic agent that can be used in patients with MG. However, it is not contraindicated and can even provide some benefits in terms of reducing muscle tone and improving respiratory function.

**Clinical Pearl / High-Yield Fact**
When managing patients with myasthenia gravis, it is essential to avoid non-depolarizing neuromuscular blocking agents like tubocurare, as they can exacerbate neuromuscular transmission and worsen muscle weakness. Always use caution and monitor closely when administering depolarizing agents like succinylcholine.

**✓ Correct Answer: B. Tubocurare**