**Core Concept**
Pre-renal azotemia occurs due to decreased renal perfusion, often as a result of hypovolemia or decreased cardiac output. This leads to activation of the renin-angiotensin-aldosterone system (RAAS), which increases sodium reabsorption in the proximal tubules, ultimately resulting in a decrease in fractional excretion of sodium (FeNa).

**Why the Correct Answer is Right**
The correct answer, **A. Urinary Na+ 1 suggests intrinsic renal failure, making this option incorrect for pre-renal azotemia.

**Option C:** Osmolality < 500 is more characteristic of intrinsic renal failure, where the kidneys are unable to concentrate or dilute urine effectively. Pre-renal azotemia typically presents with an inappropriately concentrated urine.

**Option D:** A urinary creatinine/P. creatinine ratio < 20 is not a characteristic feature of pre-renal azotemia. This ratio is more useful in assessing the completeness of urine collection.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that in pre-renal azotemia, the kidneys are working to conserve sodium and maintain blood pressure. A low urinary sodium level (< 10 mmol/L) is a critical distinguishing feature from intrinsic renal failure.

**✓ Correct Answer: A. Urinary Na+ < 10 mmol/L**