**Core Concept**
Osmotic diarrhea is a type of non-inflammatory diarrhea characterized by the presence of osmotically active substances in the gut lumen, which draw water into the lumen and cause diarrhea. This occurs due to the presence of poorly absorbed solutes in the gut, leading to an imbalance in the osmotic pressure.

**Why the Correct Answer is Right**
Osmotic diarrhea is typically caused by the ingestion of certain substances, such as lactose in lactose intolerance, sorbitol in sugar-free candies, or mannitol in certain medications. These substances are poorly absorbed by the gut and remain in the lumen, drawing water into the gut through osmosis. This leads to an increase in the amount of water in the gut, resulting in diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a true characteristic of osmotic diarrhea. The presence of osmotically active substances in the gut lumen is a hallmark of this condition.
**Option B:** This option is incorrect because it is a true cause of osmotic diarrhea. Lactose intolerance is a common cause of osmotic diarrhea in individuals who are unable to digest lactose.
**Option C:** This option is incorrect because it is a true consequence of osmotic diarrhea. Diarrhea is a common symptom of osmotic diarrhea due to the increased amount of water in the gut.

**Clinical Pearl / High-Yield Fact**
A classic exam question on osmotic diarrhea often involves identifying the substance responsible for the condition. Remember that osmotic diarrhea is caused by poorly absorbed solutes in the gut, which can include lactose, sorbitol, mannitol, and other substances.

**Correct Answer:** A.