**Core Concept**
Pre-renal azotemia, also known as prerenal azotemia, is a condition characterized by an increase in blood urea nitrogen (BUN) and serum creatinine levels due to a decrease in renal perfusion. This decrease in renal perfusion can be caused by various factors including dehydration, heart failure, and nephrotic syndrome.

**Why the Correct Answer is Right**
In pre-renal azotemia, the kidneys receive adequate blood flow, but the blood is concentrated due to dehydration or decreased cardiac output. As a result, the kidneys are unable to effectively filter waste products, leading to an increase in BUN and serum creatinine levels. The kidneys also try to conserve sodium and water by increasing the reabsorption of these substances, leading to a decrease in sodium excretion. The renin-angiotensin-aldosterone system (RAAS) is activated, leading to vasoconstriction and increased sodium reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pre-renal azotemia is not caused by a decrease in glomerular filtration rate (GFR), but rather by a decrease in renal perfusion.
**Option B:** This option is incorrect because pre-renal azotemia is not characterized by a decrease in sodium excretion, but rather an increase in sodium reabsorption.
**Option C:** This option is incorrect because pre-renal azotemia is not caused by a direct toxic effect on the kidneys, but rather by a decrease in renal perfusion.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate pre-renal azotemia from intrinsic renal causes of azotemia, such as acute tubular necrosis (ATN), by assessing the patient's volume status and renal function. A low urine sodium level ( 40 mmol/L) suggests ATN.

**Correct Answer: D.**