**Core Concept:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. Pre-operative investigations are necessary to assess the general health and potential complications that may arise during surgery.

**Why the Correct Answer is Right:** The pre-operative evaluation should include a thorough assessment of the patient's cardiovascular, respiratory, renal, and hematological systems. In the case of Down's syndrome, these investigations are important to identify any underlying health issues and ensure safe anesthesia management.

**Why Each Wrong Option is Incorrect:**

A. Stool examination (faecal occult blood test) - This is not relevant to Down's syndrome patients and focuses on gastrointestinal issues.
B. Chest x-ray - Although Down's syndrome patients may have increased risk of respiratory infections, a chest x-ray is not specifically required for Down's syndrome patients.
C. Blood urea and serum creatinine estimation - This is necessary to assess renal function but Down's syndrome patients do not necessarily have renal dysfunction, making this test irrelevant to their specific condition.
D. Electrocardiogram (ECG) - This is important in Down's syndrome patients, as they may have congenital heart defects, arrhythmias, or valvular heart diseases that require preoperative evaluation.

**Clinical Pearl:** Down syndrome patients have an increased risk of congenital heart defects, which should be addressed pre-operatively. These patients may develop complications like arrhythmias and valvular heart diseases that require preoperative evaluation with electrocardiogram (ECG).

**Correct Answer:** D. Electrocardiogram (ECG) - ECG is necessary to evaluate cardiac conduction abnormalities, arrhythmias, and congenital heart defects in Down's syndrome patients prior to surgery.