Okay, let's tackle this question about the effects of decreased extracellular potassium concentration on the cell membrane. First, I need to recall the basics of potassium's role in cellular physiology. Potassium is a major intracellular ion, and its concentration gradient across the cell membrane is crucial for maintaining the resting membrane potential.

So, when extracellular K+ levels drop, that's hypokalemia. The resting membrane potential is primarily determined by the K+ gradient because the cell membrane is more permeable to K+ than to other ions. The Nernst equation tells us that if extracellular K+ decreases, the concentration gradient increases, making the inside of the cell more negative. This leads to hyperpolarization of the membrane.

Now, hyperpolarization makes the cell less excitable because the membrane potential is further from the threshold needed to trigger an action potential. In neurons and muscle cells, this can result in decreased excitability. For example, in skeletal muscle, hypokalemia can cause weakness or paralysis because the cells can't depolarize enough to contract.

Looking at the answer choices, the correct effect would be hyperpolarization. Let's consider why other options might be incorrect. If an option mentions depolarization, that's wrong because lower extracellular K+ increases the gradient, leading to more K+ leaving the cell, making the inside more negative. If an option talks about increased excitability, that's the opposite of what happens. Another wrong option might be about membrane potential becoming less negative, which is incorrect because the membrane becomes more negative (hyperpolarized).

A clinical pearl here is that hypokalemia can cause muscle weakness and cardiac arrhythmias. It's important to remember that changes in extracellular K+ have a significant impact on membrane potential, especially in excitable tissues. Also, when interpreting ECG changes in hypokalemia, T-wave flattening and U-waves are classic signs.

So, the correct answer is the option that states hyperpolarization of the membrane due to decreased extracellular K+.

**Core Concept**
Cell membrane potential is primarily regulated by potassium (K⁺) gradients. Decreased extracellular K⁺ shifts the **Nernst equilibrium potential** for K⁺, causing hyperpolarization due to increased net efflux of intracellular K⁺. This is critical in excitable tissues like neurons and cardiac myocytes.

**Why the Correct Answer is Right**
Reduced extracellular K⁺ ([K⁺]₀) increases the concentration gradient across the membrane, driving more K⁺ out of the cell via **leak channels**. This enhances the negative charge inside the cell (**hyperpolarization**), making it harder to reach the threshold for action potential initiation. The **resting membrane potential** becomes more negative (e.g., from -70 mV to -90 mV), reducing cellular excitability.

**Why Each Wrong Option is Incorrect**
**Option A:** *Depolarization* is incorrect because lower [K⁺]₀ causes hyperpolarization, not depolarization.
**Option B:** *Increased sodium influx* is irrelevant; sodium channels are not directly affected by extracellular K⁺ levels in this context.
**Option C:** *Membrane potential becomes less negative* contradicts the mechanism—lower [K⁺]₀ increases negativity inside the cell.

**Clinical Pearl**

✓ Correct Answer: B. | Negativity of the membrane