## Core Concept
The question tests the understanding of cellular calcium homeostasis, particularly in the context of cell injury. Calcium plays a crucial role in cellular signaling and function. Under normal conditions, the cytosolic calcium concentration is tightly regulated and maintained at a low level.

## Why the Correct Answer is Right
The correct answer, , reflects the understanding that in an injured cell, the increased amounts of calcium in the cytosol primarily come from the release of calcium from the **endoplasmic reticulum (ER)**, a type of smooth endoplasmic reticulum specialized for calcium storage. The ER acts as a critical calcium reservoir, and its release of calcium into the cytosol can be triggered by various signals, including those generated by cell injury. This release is often mediated through channels like the ryanodine receptor or the inositol trisphosphate receptor (IP3R).

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because, although the plasma membrane does regulate calcium entry into the cell, the question focuses on the release of calcium from intracellular stores in response to injury.
* **Option B:** This option might seem plausible due to the involvement of the mitochondria in calcium handling; however, under conditions of cell injury, the primary and immediate source of increased cytosolic calcium is not the mitochondria but rather the ER.
* **Option D:** This option is incorrect because, in the context provided, the focus is on the rapid release of calcium from intracellular stores in response to injury, not on the uptake or storage functions typically associated with the Golgi apparatus.

## Clinical Pearl / High-Yield Fact
A critical clinical pearl is that cellular injury often leads to an influx of calcium into the cytosol, which can activate various proteases, lipases, and other enzymes, leading to cell death. This process underlies many forms of tissue injury and organ dysfunction.

## Correct Answer: C. Endoplasmic reticulum.