**Core Concept**
Acute acalculous cholecystitis is a severe inflammatory condition of the gallbladder, characterized by the absence of gallstones. This condition is often seen in critically ill patients, particularly those who are bedridden or have vascular compromise, leading to gallbladder ischemia and subsequent inflammation.

**Why the Correct Answer is Right**
In acute acalculous cholecystitis, the gallbladder is often not distended due to the underlying vascular cause, which leads to ischemia and inflammation. The distension typically associated with gallbladder inflammation is more commonly seen in cases of gallstone disease or emphysematous cholecystitis. The correct answer, **A. Distended GB**, is incorrect because a distended gallbladder is not a hallmark of acute acalculous cholecystitis.

**Why Each Wrong Option is Incorrect**
**Option B:** Vascular cause is actually a true characteristic of acute acalculous cholecystitis, as the condition is often precipitated by vascular compromise, such as shock, sepsis, or major surgery.
**Option C:** Seen in bedridden patients is also true, as these individuals are at increased risk of developing acute acalculous cholecystitis due to their compromised health status and immobility.
**Option D:** Rapid course is a characteristic feature of acute acalculous cholecystitis, which is a medical emergency requiring prompt diagnosis and treatment.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that acute acalculous cholecystitis is a clinical diagnosis, often made based on a combination of symptoms, laboratory findings, and imaging studies. A high index of suspicion is necessary to diagnose this condition, particularly in critically ill patients.

✓ Correct Answer: A. Distended GB