**Core Concept**
The sudden development of hypotension in a patient undergoing surgery, accompanied by a significant decrease in end-tidal CO2 (ETCO2), suggests a problem with ventilation or perfusion. This scenario is a classic example of a condition known as **Pulmonary Embolism (PE)**.

**Why the Correct Answer is Right**
Pulmonary embolism is a blockage of one or more pulmonary arteries by a blood clot or other material. When a large PE occurs, it can cause a sudden increase in pulmonary vascular resistance, leading to a decrease in cardiac output and subsequently a drop in blood pressure. The sudden decrease in ETCO2 is due to the reduced perfusion of the lungs, resulting in decreased CO2 removal from the blood. The body's response to this decrease in perfusion is to increase sympathetic tone, leading to tachycardia and hypertension (which can be masked by the hypotension caused by the PE).

**Why Each Wrong Option is Incorrect**
* **Option A:** A decrease in end-tidal CO2 could also be seen in conditions such as cardiac tamponade or tension pneumothorax, but these conditions would not typically present with a sudden drop in blood pressure.
* **Option B:** Hypotension and decreased ETCO2 can also be seen in conditions such as anaphylaxis, but anaphylaxis would typically present with a more rapid onset and would be accompanied by other symptoms such as bronchospasm, urticaria, and hypotension.
* **Option C:** Hypotension can be caused by a variety of factors, including anaphylaxis, cardiac tamponade, and tension pneumothorax, but these conditions would not typically present with a sudden drop in ETCO2.

**Clinical Pearl / High-Yield Fact**
In the setting of a sudden drop in ETCO2, PE should be considered a potential diagnosis, especially in patients who are at risk for thromboembolic events (e.g., those with a history of deep vein thrombosis or recent surgery). A low ETCO2 can be an early indicator of PE, and prompt recognition and treatment can improve outcomes.

**Correct Answer: C. Pulmonary Embolism**