**Core Concept**
The concept of "loss of function" refers to the fifth clinical sign added to the Galen's six signs of inflammation, which are essential for diagnosing inflammation in a patient. These signs include redness, heat, swelling, pain, loss of function, and sometimes purulent discharge.

**Why the Correct Answer is Right**
The Galen's six signs of inflammation were initially described by Galen, a Greek physician. Later, the fifth sign, "loss of function" or functio laesa, was added to the list. This sign indicates a decrease or loss of the function of the affected part, which is a crucial diagnostic indicator of inflammation. The addition of this sign helped clinicians better understand the progression of inflammation and its impact on the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the Galen's six signs of inflammation.
**Option B:** This option is incorrect because it is a different concept related to inflammation, not the fifth clinical sign.
**Option C:** This option is incorrect because it is a condition that can occur due to various reasons, not specifically the fifth clinical sign of inflammation.

**Clinical Pearl / High-Yield Fact**
Remember the Galen's six signs of inflammation as a mnemonic to diagnose inflammation: "Redness, Heat, Swelling, Pain, Loss of function, and sometimes Purulent discharge" (RHSP LoP).

**Correct Answer: C. Celsus.**