**Core Concept**
Ischemic colitis occurs when there is a reduction in blood flow to the colon, leading to damage to the mucosa and submucosa. This condition can be caused by various factors, including atherosclerosis, vasculitis, or decreased cardiac output. Radiological signs are crucial in diagnosing ischemic colitis, as they can help differentiate it from other colonic pathologies.

**Why the Correct Answer is Right**
The thumb print sign is a characteristic radiological feature of ischemic colitis. It is caused by submucosal hemorrhage and edema, which lead to thickening of the colonic wall and formation of a "thumb print" appearance on barium enema or CT scans. This sign is often seen in the left colon, particularly in the rectosigmoid region. The thumb print sign is a result of the mucosal folds becoming more pronounced due to submucosal edema and hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** The popcorn appearance is typically seen in pneumatosis intestinalis, which is a condition characterized by gas bubbles within the intestinal wall. This is not a feature of ischemic colitis.

**Option C:** The cobrahead sign is associated with sigmoid volvulus, a condition where the sigmoid colon twists around its mesenteric axis. This can lead to bowel obstruction and ischemia, but it is not a direct radiological feature of ischemic colitis.

**Option D:** The "inverted 3" sign is not a recognized radiological feature of ischemic colitis. This option may be a distractor to test the student's knowledge of radiological signs.

**Clinical Pearl / High-Yield Fact**
In addition to the thumb print sign, other radiological features of ischemic colitis include the "lead pipe" appearance of the colon, which is caused by thickening of the colonic wall, and the "cobblestone" appearance, which is caused by mucosal ulceration and edema.

**✓ Correct Answer: B. Thumb print appearance**