A 45-year-old patient presents with jaundice, right-sided abdominal pain, dark urine and pale stools.

The lack of which of the following is the likely cause of this patient's pale stools?

Options:
explanation: This is not the correct answer. While albumin is the major plasma protein that bilirubin binds to in the bloodstream to be transported to the liver for conjugation, a lack of will not necessarily imply a decrease in bilirubin conjugation and subsequent excretion. More specifically, stercobilin is the brown pigment responsible for the colour of stool and a lack of stercobilin will result in pale stools.
name: Albumin
label: e
answer: no
explanation: This is not the correct answer. Stercobilin is the brown pigment found in faeces, hence responsible for the colour. In hepatic and post-hepatic jaundice, there is a mixed (unconjugated and conjugated) hyperbilirubinaemia, and conjugated hyperbilirubinaemia respectively. Either way, conjugated bilirubin is water-soluble and can be excreted via the urine, while unconjugated bilirubin cannot. Consequently, a dark "coca-cola" urine can be seen in mixed or conjugated disease, accompanied by pale stools due to reduced levels of stercobilin entering the gastrointestinal tract. As such an excess of conjugated bilirubin often presents with pale stool, however, a lack of is unlikely to cause reduced levels of stercobilin in stools.
name: Conjugated bilirubin
label: c
answer: no
explanation: This is not the correct answer. Bilirubin is the normal breakdown product formed from the destruction of red blood cells (catabolism of haem). It undergoes conjugation in the liver forming conjugated bilirubin (water-soluble), and is then excreted via bile into the gastrointestinal tract. The majority of it is egested in the faeces as urobilinogen and stercobilin, where the latter is a breakdown product of urobilinogen. 10% of urobilinogen is reabsorbed in the bloodstream and excreted through the kidneys. Urobilinogen is colourless while stercobilin is the brown pigment responsible for the colour.
name: Urobilinogen
label: b
answer: no
explanation: This is not the correct answer. Unconjugated hyperbilirubinemia is seen in pre-hepatic jaundice where there is excess breakdown of red blood cells, and this presents with normal/ darker stool colour as stercobilin still enters the GI tract. In contrast, a lack of unconjugated bilirubin will not necessarily imply lower levels of stercobilin in stool and consequently, pale stools.
name: Unconjugated bilirubin
label: d
answer: no
explanation: This is the correct answer. Bilirubin is the normal breakdown product formed from the destruction of red blood cells (catabolism of haem). It undergoes conjugation in the liver forming conjugated bilirubin (water-soluble), and is then excreted via bile into the gastrointestinal tract. The majority of it is egested in the faeces as urobilinogen and stercobilin, where the latter is a breakdown product of urobilinogen. 10% of urobilinogen is reabsorbed in the bloodstream and excreted through the kidneys. Stercobilin is the brown pigment found in faeces, hence responsible for the colour. As such, a lack of stercobilin will result in the patient's pale stools.
name: Stercobilin
label: a
answer: yes

Correct answer: a