Okay, let's tackle this question. The question is about free water clearance and what decreases it. First, I need to remember what free water clearance means. Free water clearance (CH2O) is the volume of plasma ultrafiltrated per minute that is free of solute. It's calculated as urine flow minus osmolar clearance. So, when CH2O is negative, it means the body is retaining free water, which happens in conditions like SIADH where there's excess ADH causing water retention.

The question asks which factor decreases free water clearance. Let's think about the options. If the options include ADH, that's a key point. ADH (antidiuretic hormone) increases water reabsorption in the collecting ducts by inserting aquaporin-2 channels. This reduces free water excretion, thus decreasing CH2O. So, ADH administration would decrease free water clearance.

Other options might include things like diuretics, but loop diuretics increase free water clearance by reducing osmotic clearance. Thiazides might have a different effect. Also, conditions like diabetes insipidus (central or nephrogenic) would increase free water clearance because there's a lack of ADH. So if an option is lack of ADH, that's incorrect.

Wait, the correct answer should be ADH. Let me confirm the calculation. Free water clearance = V - Cosm. ADH increases Cosm by concentrating urine, so if Cosm increases and V is maintained, then CH2O decreases. Alternatively, if ADH causes more water reabsorption, the urine output (V) might decrease, but if the osmolality is higher, the Cosm is higher. Either way, CH2O would be lower.

So, the correct answer is ADH. Now, the other options: if they include things like thiazides, loop diuretics, or maybe alcohol, which inhibits ADH, then those would increase free water clearance. So if an option is ADH, it's correct. If the options include other hormones or drugs, they should be incorrect.

Clinical pearl: ADH causes water retention, so it decreases free water clearance. Remember that negative free water clearance means the body is retaining water, as in SIADH. High-yield fact: ADH's role in aquaporins and its effect on free water clearance.

**Core Concept**
Free water clearance (CH₂O) measures the volume of plasma ultrafiltrate that is free of solute. It is calculated as urine flow (V) minus osmolar clearance (Cosm). Decreased CH₂O occurs when the body retains free water, typically due to antidiuretic hormone (ADH) action.

**Why the Correct Answer is Right**
ADH (vasopressin) increases water reabsorption in the renal collecting ducts by inserting aquaporin-2 channels into the apical membrane. This reduces free water excretion, lowering CH₂O. In states of ADH excess (e.g., SIADH), urine becomes hyperosmolar, and CH₂O becomes negative, indicating net water retention.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiazide diuretics increase free water clearance by reducing osmotic clearance via sodium and water reabsorption.
**Option

✓ Correct Answer: D. Furosemide