**Core Concept**
The body's metabolic response to protein intake involves the breakdown of amino acids into various products, including carbon dioxide, water, and nitrogen. This process is influenced by the body's protein requirements, dietary protein intake, and the efficiency of protein utilization.

**Why the Correct Answer is Right**
When 10 g of protein is metabolized, the body utilizes the amino acids to synthesize new proteins, generate energy, or excrete nitrogen waste. Approximately 10-15% of the nitrogen from the protein is converted to urea, which is then excreted by the kidneys. The remaining nitrogen is either used for protein synthesis or converted to ammonia. The carbon skeletons from the amino acids are either used for energy production or converted to glucose through gluconeogenesis. The energy yield from the metabolism of 10 g of protein is approximately 40-50 kcal.

**Why Each Wrong Option is Incorrect**
**Option A:** This is an incorrect representation of the energy yield from protein metabolism.
**Option B:** This is an overestimation of the nitrogen excreted as urea.
**Option C:** This is an underestimation of the energy yield from protein metabolism.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the body's protein requirements vary depending on factors such as age, sex, and nutritional status. A balanced diet that includes adequate protein intake is crucial for maintaining optimal health and preventing protein deficiency.

**Correct Answer: B. 40-50 kcal**