**Core Concept**
Shock is a life-threatening condition characterized by inadequate tissue perfusion, leading to cellular hypoxia and eventually organ dysfunction. In shock, the body's compensatory mechanisms are activated to maintain blood pressure and ensure perfusion of vital organs.

**Why the Correct Answer is Right**
During shock, the kidneys experience decreased perfusion, leading to a decrease in glomerular filtration rate (GFR). This is due to the reduced blood flow and pressure in the renal arteries. The decreased GFR is a hallmark of shock and can be used as an indicator of the severity of the condition. Additionally, the renin-angiotensin-aldosterone system (RAAS) is activated in shock to increase blood pressure and maintain perfusion. This results in **increased renin** levels, which stimulate the release of aldosterone and lead to vasoconstriction and increased blood pressure.

**Why Each Wrong Option is Incorrect**
* **Option B:** Increased renin is a correct response in shock, so this option is incorrect.
* **Option C:** Decreased renin is incorrect because renin levels typically increase in shock to activate the RAAS.
* **Option D:** Decreased cortisol is not a direct feature of shock. Cortisol levels may actually increase in response to stress and shock.
* **Option A:** Decreased GFR is a feature of shock, but increased renin is also a correct response, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
In shock, the body's compensatory mechanisms can lead to increased renin and aldosterone levels, resulting in vasoconstriction and increased blood pressure. However, this can also lead to decreased renal perfusion and GFR.

**Correct Answer: A. Decreased GFR**