**Core Concept**
Post-obstructive diuresis is a condition characterized by excessive urine production following relief of urinary tract obstruction, often due to benign prostatic hypertrophy. This condition results from the sudden release of large volumes of urine stored in the bladder, leading to hypovolemia and subsequent activation of the renin-angiotensin-aldosterone system (RAAS).

**Why the Correct Answer is Right**
The correct answer is related to the expected changes in urine electrolyte levels in post-obstructive diuresis. In this condition, there is an increase in the reabsorption of sodium and water, resulting in a decrease in the sodium excretion rate. This is mediated by the activation of the RAAS, leading to increased aldosterone levels, which in turn promotes sodium reabsorption in the collecting ducts of the kidneys.

**Why Each Wrong Option is Incorrect**

* **Option A:** A low sodium concentration in the urine is not expected in post-obstructive diuresis, as the kidneys are trying to conserve sodium and water to compensate for the hypovolemia.
* **Option B:** A high potassium concentration in the urine is not a characteristic feature of post-obstructive diuresis, as the condition is more related to sodium and water handling.
* **Option C:** A low osmolality in the urine is not expected in post-obstructive diuresis, as the kidneys are trying to concentrate the urine to conserve water.

**Clinical Pearl / High-Yield Fact**
In post-obstructive diuresis, it's essential to monitor the patient's fluid status closely and administer fluids and electrolytes as needed to prevent hypovolemia and electrolyte imbalances.

**Correct Answer: C.**