Pulmonary Embolism: This is a common and serious complication that can occur after surgery, especially on the third postoperative day. Immobility, surgical trauma, and potential hypercoagulability increase the risk of deep vein thrombosis and subsequent pulmonary embolism.

Surgical Site Infection: Infections can start to manifest around this time frame, especially if there was contamination during surgery or if the patient has risk factors for infection.
Pneumonia: Postoperative pneumonia is a significant concern, particularly if the patient underwent abdominal or thoracic surgery, has been intubated, or has limited mobility.
Urinary Tract Infection: Catheter-associated urinary tract infections can occur, especially if a urinary catheter was used during or after surgery.
Adhesive Ileus or Bowel Obstruction: These conditions can cause significant abdominal pain, nausea, vomiting, and constipation, and are more common after abdominal surgery.

Myocardial Infarction: Although less common, myocardial infarction can occur postoperatively, especially in patients with cardiovascular risk factors. It requires immediate attention.
Sepsis: Any sign of infection or organ dysfunction should prompt a workup for sepsis, as it is life-threatening and requires prompt intervention.
Anastomotic Leak: A serious complication of certain surgeries (like gastrointestinal surgery), anastomotic leaks can lead to peritonitis and sepsis if not recognized and treated promptly.

Fat Embolism Syndrome: More common after orthopedic surgeries, especially those involving long bone fractures, fat embolism syndrome can present with respiratory distress, neurological symptoms, and petechial rash.
Acute Respiratory Distress Syndrome (ARDS): A severe lung condition that can arise from various causes, including infection, trauma, or inhalation injury, ARDS is rare but critical and requires intensive care.
Thrombocytopenia-associated Multiple Organ Failure: A rare but potentially fatal condition that can occur postoperatively, often associated with heparin-induced thrombocytopenia or other causes of severe thrombocytopenia.