**Core Concept**
The 'Egg on Side' appearance is a radiological sign that describes the appearance of a specific type of abdominal collection. This sign is typically seen on imaging studies, such as ultrasound or CT scans, and is characterized by the collection having a thin, walled appearance with a thickened, irregular wall.

**Why the Correct Answer is Right**
The 'Egg on Side' appearance is most commonly associated with a splenic abscess. A splenic abscess is a collection of pus within the spleen, often resulting from a bacterial infection. The abscess can cause the spleen to become enlarged and can lead to a range of symptoms, including pain, fever, and fatigue. The 'Egg on Side' appearance is thought to be due to the thickened, irregular wall of the abscess, which can be visualized on imaging studies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the 'Egg on Side' appearance is not typically associated with a hepatic abscess, which would more likely have a smooth, rounded appearance.
**Option B:** This option is incorrect because a simple ovarian cyst would not have the characteristic 'Egg on Side' appearance, as it would not have a thickened, irregular wall.
**Option C:** This option is incorrect because a pancreatic pseudocyst would not have the same appearance as a splenic abscess, as it would not have the same type of thickened, irregular wall.

**Clinical Pearl / High-Yield Fact**
It's essential for radiologists to recognize the 'Egg on Side' appearance, as it can be a critical clue in diagnosing a splenic abscess. This sign can help differentiate a splenic abscess from other types of abdominal collections and guide further management.

**Correct Answer: D. Splenic abscess.**