FRCS Surgical Physiology

Question 1

A patient loses 1500 ml of blood acutely. According to the ATLS classification, this represents which class of haemorrhagic shock?

Accepted Answer

Class III

Answer

Class III haemorrhage represents 30-40% blood volume loss (approximately 1500-2000 ml in a 70 kg adult). It is characterised by tachycardia, hypotension, tachypnoea, reduced urine output, and altered mental status. Crystalloid and blood transfusion are typically required.

Question 2

In the fasting state, which organ is the primary consumer of ketone bodies for energy?

Accepted Answer

Brain

Answer

During prolonged fasting, the brain adapts to utilise ketone bodies (beta-hydroxybutyrate and acetoacetate) for up to 70% of its energy needs. The liver produces ketone bodies but cannot utilise them as it lacks the enzyme succinyl-CoA:3-ketoacid CoA transferase. Red blood cells lack mitochondria and rely solely on glycolysis.

Question 3

A patient with a large bowel obstruction develops metabolic acidosis. Which acid-base disturbance is most likely if the patient also has persistent vomiting?

Accepted Answer

Mixed metabolic acidosis and metabolic alkalosis

Answer

Large bowel obstruction causes metabolic acidosis from ischaemia, bacterial translocation, and lactate production. Concurrent vomiting causes loss of gastric hydrochloric acid, producing metabolic alkalosis. The combination results in a mixed acid-base disturbance. Arterial blood gas analysis and calculation of the anion gap help differentiate the components.

Question 4

Which of the following is the most important buffer system in the extracellular fluid?

Accepted Answer

Bicarbonate-carbonic acid system

Answer

The bicarbonate-carbonic acid (HCO3-/H2CO3) system is the principal extracellular buffer. It is an open system: CO2 is regulated by the lungs and HCO3- by the kidneys, giving it enormous buffering capacity. Although the phosphate system is important intracellularly and in renal tubular fluid, and haemoglobin is the main intracellular buffer of red blood cells, bicarbonate dominates extracellularly.

Question 5

Following major abdominal surgery, a patient develops oliguria with a urine sodium concentration of 8 mmol/L. What is the most likely cause?

Accepted Answer

Appropriate physiological response to surgery (ADH and aldosterone secretion)

Answer

A low urine sodium (<20 mmol/L) indicates avid sodium reabsorption by the kidneys, consistent with a pre-renal cause. Post-surgical stress triggers ADH and aldosterone release, causing sodium and water retention. In acute tubular necrosis, the urine sodium would typically be >40 mmol/L as the damaged tubules lose their ability to reabsorb sodium.

FRCS Part A Practice Test

FRCS Part A Practice Test

FRCS Surgical Physiology