**Core Concept**
Kehr's sign is a clinical sign associated with the presence of bleeding in the spleen, which refers to the referred pain felt in the left shoulder due to the irritation of the diaphragm by blood in the peritoneal cavity.

**Why the Correct Answer is Right**
When blood accumulates in the peritoneal cavity due to splenic rupture, it can irritate the diaphragm, leading to pain referred to the left shoulder. This phenomenon occurs because the phrenic nerve, which innervates the diaphragm, also carries sensory fibers that can transmit pain signals to the brain. The diaphragm is closely related to the left shoulder area, resulting in referred pain.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to Kehr's sign, as it does not describe the pathophysiology of referred pain in the left shoulder.
* **Option B:** While hematemesis (vomiting blood) can be a symptom of upper gastrointestinal bleeding, it is not directly related to Kehr's sign, which is specific to splenic rupture.
* **Option C:** The McBurney sign is a clinical indicator of appendicitis, which is a different condition altogether.

**Clinical Pearl / High-Yield Fact**
Kehr's sign is an important diagnostic clue for splenic rupture, especially in patients who present with left upper quadrant abdominal pain and no obvious signs of trauma.

**Correct Answer:** C.