**Question:** Which of the following is considered the active form of calcium?

A. Calcium ion (Ca²⁺)
B. Sodium calcium exchanger (NCX)
C. Sodium-calcium exchanger (NCX)
D. Parathyroid hormone (PTH)

**Core Concept:** The active form of calcium refers to the ionized calcium (Ca²⁺) which is the biologically active form of calcium that can diffuse through cell membranes and participate in physiological processes. Calcium ions are the primary regulators of various cellular functions, including muscle contraction, nerve transmission, and enzyme activation.

**Why the Correct Answer is Right:** The correct answer is Calcium ion (Ca²⁺) because it is the bioavailable form of calcium that can interact with target cells and proteins. Calcium ions are essential for various physiological processes, such as muscle contraction, nerve transmission, and coagulation.

**Why Each Wrong Option is Incorrect:**

B. Sodium calcium exchanger (NCX) and C. Sodium-calcium exchanger (NCX) are proteins involved in maintaining calcium homeostasis by exchanging sodium ions for calcium ions across cell membranes. However, they are not the active form of calcium itself.

D. Parathyroid hormone (PTH) regulates blood calcium levels by stimulating osteoclasts to release calcium from bones into the bloodstream. PTH does not directly influence the active form of calcium.

**Clinical Pearl:** The active form of calcium is crucial to understand as it enables us to interpret laboratory results, diagnose conditions like hypocalcemia and hypercalcemia, and assess the effectiveness of calcium-regulating medications and treatments.

**Correct Answer:** Calcium ion (Ca²⁺) is the active form of calcium involved in physiological processes.