**Core Concept**
Gastrointestinal (GI) motility refers to the movement of food, fluid, and gas through the digestive tract. It is regulated by a complex interplay of hormones, nerves, and muscles. In this context, decreased GI motility can lead to symptoms such as bloating, abdominal pain, and difficulty passing stool.

**Why the Correct Answer is Right**
The correct answer is a condition that does not typically decrease GI motility. Conditions that decrease GI motility often involve damage to the enteric nervous system, hormonal imbalances, or abnormal muscle function. For example, diabetes can damage the enteric nervous system, leading to gastroparesis, a condition characterized by delayed gastric emptying. Similarly, conditions such as scleroderma can cause fibrosis and atrophy of the GI tract, leading to decreased motility.

**Why Each Wrong Option is Incorrect**
**Option A:** Diabetes mellitus - Decreased GI motility is a common complication of diabetes, particularly gastroparesis.
**Option B:** Scleroderma - This autoimmune disease can cause fibrosis and atrophy of the GI tract, leading to decreased motility.
**Option C:** Parkinson's disease - This neurodegenerative disorder can cause decreased GI motility due to damage to the enteric nervous system.

**Clinical Pearl / High-Yield Fact**
Conditions that affect the enteric nervous system, such as diabetes and Parkinson's disease, can lead to decreased GI motility. It's essential to consider these conditions in patients presenting with symptoms of gastroparesis or slowed GI transit.

**Correct Answer: D.**