**Core Concept**
Hypokalemic periodic paralysis is a rare genetic disorder characterized by recurrent episodes of muscle weakness and paralysis, often triggered by potassium fluctuations. This condition involves abnormalities in ion channels that regulate muscle membrane excitability.

**Why the Correct Answer is Right**
The correct answer is D. Ca2+, as hypokalemic periodic paralysis is primarily caused by mutations in the calcium channel gene (CACNA1S). These mutations lead to abnormal calcium channel function, resulting in excessive calcium entry into muscle cells during depolarization. This calcium influx triggers muscle hyperexcitability, ultimately leading to muscle weakness and paralysis. The condition is often triggered by potassium fluctuations, which can shift the calcium channel's voltage dependence, leading to abnormal calcium entry.

**Why Each Wrong Option is Incorrect**
**Option A:** K+ is not directly involved in hypokalemic periodic paralysis, as the condition is not caused by a deficiency of potassium channels or channels regulating potassium flux.

**Option B:** Na+ channels are not primarily affected in hypokalemic periodic paralysis. While sodium channels are crucial for muscle excitability, the condition is specifically linked to calcium channel dysfunction.

**Option C:** Cl- channels are not directly implicated in hypokalemic periodic paralysis. Chloride channels play a role in maintaining muscle membrane potential, but the condition is primarily caused by abnormalities in calcium channels.

**Clinical Pearl / High-Yield Fact**
Hypokalemic periodic paralysis can be triggered by factors that shift the calcium channel's voltage dependence, such as potassium fluctuations, caffeine, or certain medications like beta-agonists. These triggers can lead to the characteristic episodes of muscle weakness and paralysis.

**✓ Correct Answer: D. Ca2+ Calcium channels are primarily affected in hypokalemic periodic paralysis.**