**Core Concept**
Osmotic diarrhea occurs when undigested carbohydrates, proteins, or other osmotically active substances are present in the small intestine, leading to an imbalance in the osmotic gradient and an influx of water into the intestinal lumen. This type of diarrhea is characterized by a high volume of watery stools.

**Why the Correct Answer is Right**
Lactase deficiency is a condition where the small intestine lacks sufficient lactase enzyme to break down lactose, a disaccharide found in milk and other dairy products. Undigested lactose remains in the small intestine, drawing water into the intestinal lumen through osmosis, resulting in osmotic diarrhea. This is a classic example of osmotic diarrhea, as lactose is an osmotically active substance that cannot be absorbed by the small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** Ulcerative sprue, also known as celiac disease, is an autoimmune disorder that causes inflammation and damage to the small intestine, leading to malabsorption of nutrients. While it can cause diarrhea, it is not typically associated with osmotic diarrhea.

**Option B:** Surreptitious ingestion of laxatives is a cause of secretory diarrhea, not osmotic diarrhea. Laxatives work by stimulating the intestinal muscles to contract and move contents through the gut, rather than by disrupting the osmotic balance.

**Option C:** Endocrine tumors, such as those secreting vasoactive intestinal peptide (VIPoma), can cause secretory diarrhea due to the excessive production of hormones that stimulate intestinal secretion. This is not typically associated with osmotic diarrhea.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with diarrhea, it is essential to consider the mechanism of diarrhea, as this can guide further investigation and treatment. Osmotic diarrhea is often associated with malabsorption or the presence of undigested carbohydrates or proteins, while secretory diarrhea is typically caused by hormonal imbalances or other systemic disorders.

**✓ Correct Answer: A. Lactase deficiency**