**Core Concept**
Diabetic ketoacidosis (DKA) is a life-threatening complication of type 1 diabetes characterized by hyperglycemia, ketone production, and metabolic acidosis. A key clinical feature is severe fluid loss due to osmotic diuresis, leading to significant dehydration.

**Why the Correct Answer is Right**
In DKA, high blood glucose levels cause osmotic diuresis, where the kidneys excrete large amounts of glucose and water. This results in marked fluid loss, leading to dehydration. Dehydration manifests as dry mucous membranes, decreased skin turgor, and reduced urine output. It is a hallmark and early sign of DKA, often preceding other symptoms like nausea or confusion.

**Why Each Wrong Option is Incorrect**
Option A: Loss of sweating is not a reliable or consistent sign of DKA. While autonomic dysfunction can occur, it is not a primary or diagnostic feature.
Option B: Depression can occur in DKA due to metabolic derangements, but it is not a specific or hallmark sign. It is more common in other conditions like hypoglycemia or encephalopathy.
Option D: Absent deep tendon reflexes are associated with hyperkalemia or hypocalcemia, which may occur in DKA, but they are not a primary or specific sign. They are more commonly linked to hypocalcemia in hyperkalemia or hypomagnesemia.

**Clinical Pearl / High-Yield Fact**
In DKA, dehydration is a primary and early clinical sign due to osmotic diuresis. Always assess for dry mouth, decreased skin turgor, and reduced urine output—these are key indicators of fluid loss and help in early recognition.

✓ Correct Answer: C. Dehydration