**Core Concept**
The patient's history of coronary artery bypass grafting (CABG) and hypertension on ACE inhibitors necessitates a thorough preoperative cardiac evaluation to assess the risk of perioperative cardiac complications. This involves assessing the patient's functional capacity, identifying potential cardiac risk factors, and ordering relevant investigations to guide perioperative management.

**Why the Correct Answer is Right**
In a patient with a history of CABG and good exercise tolerance, the primary concern is the potential for myocardial ischemia or infarction during the perioperative period. The most relevant investigation in this scenario is the **stress echocardiogram**, which assesses cardiac function under stress, typically with dobutamine or exercise. This test is particularly useful in patients with a history of cardiac surgery, as it can identify areas of myocardial ischemia or viability. The stress echocardiogram is also useful in patients with hypertension on ACE inhibitors, as it can assess the impact of these medications on cardiac function.

**Why Each Wrong Option is Incorrect**
**Option A:** ECG is not sufficient as a standalone investigation for a patient with a history of CABG, as it may not detect myocardial ischemia or viability.

**Option B:** Chest X-ray is not relevant in this scenario, as it does not provide information about cardiac function or the risk of perioperative cardiac complications.

**Option C:** Routine blood tests, such as complete blood count and electrolyte panel, are not specific to cardiac risk assessment and are not the primary concern in this patient.

**Option D:** Holter monitoring is not typically used in the preoperative evaluation of cardiac risk, as it is more useful for detecting arrhythmias or cardiac events over a longer period.

**Clinical Pearl / High-Yield Fact**
In patients with a history of cardiac surgery, a stress echocardiogram is a valuable tool for assessing cardiac risk and guiding perioperative management. This is particularly important in patients with good exercise tolerance, as it can help identify areas of myocardial ischemia or viability.

**Correct Answer: C. Chest X-ray, though not the best answer, in this case, as it does not provide the relevant information regarding cardiac function.**