**Core Concept**
The patient's sudden decrease in end-tidal carbon dioxide (ETCO2) and subsequent hypotension and desaturation during surgery under general anesthesia may indicate a critical issue with her respiratory or cardiovascular system. The decrease in ETCO2 can be due to various factors, including hypoventilation, decreased cardiac output, or a combination of both.

**Why the Correct Answer is Right**
The most probable cause of the patient's condition is a pulmonary embolism (PE). Pulmonary embolism can occur in a patient who is immobile, such as during surgery, and can be exacerbated by factors such as obesity and hypertension. The sudden decrease in ETCO2 may indicate a decrease in cardiac output, as the lungs are not receiving adequate blood flow to exchange carbon dioxide. The subsequent hypotension and desaturation can be due to the decreased cardiac output and subsequent ischemia to vital organs.

**Why Each Wrong Option is Incorrect**
**Option A:** **Hypoventilation** - While hypoventilation can cause a decrease in ETCO2, it would not typically cause a sudden drop from 40 to 20 mmHg, nor would it explain the subsequent hypotension and desaturation.

**Option B:** **Anesthetic agent overdose** - Anesthetic agent overdose can cause respiratory depression, but it would not typically cause a sudden drop in ETCO2, nor would it explain the subsequent hypotension and desaturation.

**Option C:** **Myocardial infarction** - Myocardial infarction can cause a decrease in cardiac output, but it would not typically cause a sudden drop in ETCO2, nor would it explain the subsequent desaturation.

**Clinical Pearl / High-Yield Fact**
A sudden drop in ETCO2 during surgery should prompt immediate investigation for a pulmonary embolism, as it can be a life-threatening condition. Remember the "4 Ps" of PE: **P**ain, **P**alpitations, **P**anic, and **P**ulselessness.

**Correct Answer:** D.