**Core Concept**
Postural hypotension, also known as orthostatic hypotension, is a sudden drop in blood pressure that occurs when an individual stands up from a sitting or lying down position. This condition is often caused by autonomic dysfunction, which affects the body's ability to regulate blood pressure.

**Why the Correct Answer is Right**
The most common cause of postural hypotension is a decrease in blood volume due to autonomic dysfunction, particularly affecting the sympathetic nervous system's ability to constrict blood vessels. This results in a lack of sufficient blood pressure increase upon standing, leading to a drop in blood pressure. The reduction in blood volume can be caused by various factors, including dehydration, excessive diuretic use, or autonomic nervous system disorders such as pure autonomic failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Not the most common cause, as it is less frequently encountered in clinical practice.
**Option B:** While diabetes can cause autonomic neuropathy, it is not the most common cause of postural hypotension.
**Option C:** Not the primary cause, as it is more related to acute blood loss or severe dehydration.

**Clinical Pearl / High-Yield Fact**
Be aware that postural hypotension can be a sign of an underlying autonomic nervous system disorder, such as pure autonomic failure, which requires prompt medical evaluation and treatment.

**Correct Answer: D. Decrease in blood volume due to autonomic dysfunction.**