**Core Concept**
Small intestinal obstruction is a mechanical or non-mechanical blockage of the small intestine, which can lead to severe complications if not promptly treated. The causes of small intestinal obstruction can be broadly classified into mechanical and non-mechanical types, with mechanical obstruction being more common.

**Why the Correct Answer is Right**
The most common cause of small intestinal obstruction is adhesions, which are bands of fibrous tissue that form between loops of intestine or between the intestine and other abdominal structures. These adhesions can be a result of previous abdominal surgery, inflammation, or other conditions. They can lead to bowel twisting, kinking, or complete obstruction of the intestine. Adhesions can also lead to bowel ischemia, which is a medical emergency requiring prompt surgical intervention. Other mechanical causes of small intestinal obstruction include hernias, volvulus, intussusception, and tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Volvulus is a cause of small intestinal obstruction, but it is less common than adhesions.
**Option B:** Intussusception is a cause of small intestinal obstruction, but it is more commonly seen in children.
**Option C:** Tumors can cause small intestinal obstruction, but they are a less common cause compared to adhesions.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that mechanical causes of small intestinal obstruction are more common than non-mechanical causes, and adhesions are the leading cause of mechanical obstruction. Prompt diagnosis and treatment are crucial to prevent complications and improve patient outcomes.

**Correct Answer:** A. Adhesions.