Tetraethyl ammonium acts by blocking which of the following channels?

**Core Concept**
The question tests the understanding of **ion channel physiology**, specifically the action of **tetraethyl ammonium (TEA)** on various channels. TEA is a known blocker of certain ion channels, affecting neural and muscular function.

**Why the Correct Answer is Right**
Tetraethyl ammonium acts by blocking **potassium channels**. This action is crucial in understanding its effects on **nerve conduction** and **muscle contraction**. By blocking potassium channels, TEA reduces the outward flow of potassium ions, thereby affecting the repolarization phase of the action potential.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because TEA's primary action is not on sodium channels.
**Option B:** Incorrect as TEA does not primarily act on calcium channels.
**Option D:** Incorrect because, although TEA has various effects, its primary described action in this context is not on chloride channels.

**Clinical Pearl / High-Yield Fact**
Remember, TEA's blockade of potassium channels can lead to **prolonged depolarization**, affecting both nerve and muscle function. This is a key point in understanding its pharmacological effects.

**Correct Answer:** D. Potassium channels.