**Core Concept**
In the immediate post-operative period, respiratory insufficiency can arise due to various factors, primarily related to the surgical procedure, anesthesia, and patient comorbidities. One of the key concepts is the impact of surgical stress on the respiratory system, including changes in lung mechanics and gas exchange.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of post-operative respiratory insufficiency. This condition often results from atelectasis, which is the collapse or closure of a lung resulting in reduced or absent gas exchange. Atelectasis can be caused by factors such as mechanical ventilation, anesthesia-induced respiratory depression, and pain leading to shallow breathing. Additionally, post-operative patients may experience a decrease in functional residual capacity (FRC) due to surgical stress, leading to increased respiratory rate and decreased oxygenation.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypoxemia is a common cause of respiratory insufficiency in the post-operative period, resulting from factors such as atelectasis, pneumonia, or respiratory depression. This option is not incorrect and is actually a common cause.

**Option B:** Pneumonia is a well-known complication of post-operative respiratory insufficiency, often caused by aspiration of food or gastric contents during anesthesia. This option is not incorrect and is a valid cause.

**Option C:** Pulmonary embolism is a potential cause of respiratory insufficiency in the post-operative period, particularly in patients with a history of deep vein thrombosis or those undergoing major surgery. This option is not incorrect and is a valid cause.

**Option D:** Since all the options A, B, and C are common causes of respiratory insufficiency, this makes D the correct answer.

**Clinical Pearl / High-Yield Fact**
It is essential for post-operative care teams to monitor patients closely for signs of respiratory insufficiency, including increased respiratory rate, decreased oxygen saturation, and decreased lung sounds. Early recognition and treatment of these complications can significantly improve patient outcomes.

**Correct Answer: D. All of the above.**