Question 256#

A small study is designed to look at the link between drinking alcohol and liver cirrhosis. One hundred patients with liver cirrhosis were questioned and it was found that 80 of them drank excessive alcohol. As a control, one hundred patients without liver cirrhosis were questioned and only 20 of these patients drank excessively.

What is the odds ratio of developing liver cirrhosis for people who drink excessively compared to those who do not?

A. 2

Correct Answer D:

The odds of a patient with liver cirrhosis having a history of excessive drinking is 80/20 = 4. The odds of a patient without liver cirrhosis having a history of excessive drinking is 20/80 = 0.25. Therefore the odds ratio = 4 / 0.25 = 16

Odds and odds ratio:

Odds are a ratio of the number of people who incur a particular outcome to the number of people who do not incur the outcome. The odds ratio may be defined as the ratio of the odds of a particular outcome with experimental treatment and that of control.

Odds ratios are the usual reported measure in case-control studies. It approximates to relative risk if the outcome of interest is rare.

For example, if we look at a trial comparing the use of paracetamol for dysmenorrhoea compared to placebo we may get the following results:

The odds of achieving significant pain relief with paracetamol = 40 / 20 = 2

The odds of achieving significant pain relief with placebo = 30 / 60 = 0.5

Therefore the odds ratio = 2 / 0.5 = 4

Question 257#

Cystic fibrosis is due to a defect in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.

Which chromosome is this gene located on?

A. Chromosome 3

Correct Answer B:

Cystic fibrosis (CF) is an autosomal recessive disorder causing increased viscosity of secretions (e.g. lungs and pancreas). It is due to a defect in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which codes a cAMP-regulated chloride channel.

In the UK 80% of CF cases are due to delta F508 on the long arm of chromosome 7. Cystic fibrosis affects 1 per 2500 births, and the carrier rate is c. 1 in 25.

Organisms which may colonize CF patients:

• Staph aureus
• Pseudomonas aeruginosa
• Burkholderia cepacia*
• Aspergillus

*Previously known as Pseudomonas cepacia.

Question 258#

A 54-year-old woman is admitted to the Emergency Department following what sounds like an episode of vasovagal syncope. Blood gases on admission show a metabolic acidosis.

Blood tests are reported as follows:

• Na⁺ 143 mmol/l
• K⁺ 3.0 mmol/l
• Chloride 116 mmol/l
• Bicarbonate 18 mmol/l
• Urea 4.0 mmol/l
• Creatinine 88 μmol/l

Which one of the following is most likely to explain the metabolic acidosis?

A. Lithium overdose

Correct Answer E:

The anion gap is normal, (143 + 3.0) - (116 + 18) = 12 mmol/l, which is consistent with a ureterosigmoidostomy. 

Aspirin overdose, myocardial infarction and alcoholic ketoacidosis would cause a raised anion gap.

Metabolic acidosis:

Metabolic acidosis is commonly classified according to the anion gap. This can be calculated by: (Na⁺ + K⁺) - (Cl- + HCO-3). If a question supplies the chloride level then this is often a clue that the anion gap should be calculated. The normal range = 10-18 mmol/L

Normal anion gap ( = hyperchloraemic metabolic acidosis):

• Gastrointestinal bicarbonate loss: diarrhoea, ureterosigmoidostomy, fistula
• Renal tubular acidosis
• Drugs: e.g. acetazolamide
• Ammonium chloride injection
• Addison's disease

Raised anion gap:

• Lactate: shock, hypoxia
• ketones: diabetic ketoacidosis, alcohol
• Urate: renal failure
• Acid poisoning: salicylates, methanol 

Metabolic acidosis secondary to high lactate levels may be subdivided into two types:

• Lactic acidosis type A: shock, hypoxia, burns
• Lactic acidosis type B: metformin

Question 259#

Which one of the following statements regarding the normal menstrual cycle is incorrect?

A. A number of follicles develop in the follicular phase under the influence of FSH

Correct Answer D:

LH surge causes ovulation.

Menstrual cycle The menstrual cycle may be divided into the following phases:

Further details are given in the table below:

Question 260#

You are advising a patient who has recently been diagnosed with chronic kidney disease stage 4 with regards to her diet.

Which one of the following foods should she eat in moderation due to the high potassium content?

A. Tomatoes

Correct Answer A:

Hyperkalaemia:

Plasma potassium levels are regulated by a number of factors including aldosterone, acid-base balance and insulin levels. Metabolic acidosis is associated with hyperkalaemia as hydrogen and potassium ions compete with each other for exchange with sodium ions across cell membranes and in the distal tubule. ECG changes seen in hyperkalaemia include tall-tented T waves, small P waves, widened QRS leading to a sinusoidal pattern and asystole.

Causes of hyperkalaemia:

• Acute renal failure
• Drugs*: potassium sparing diuretics, ACE inhibitors, angiotensin 2 receptor blockers, spironolactone, ciclosporin, heparin**
• Metabolic acidosis
• Addison's
• Rhabdomyolysis
• Massive blood transfusion

Foods that are high in potassium:

• Salt substitutes (i.e. Contain potassium rather than sodium)
• Bananas, oranges, kiwi fruit, avocado, spinach, tomatoes

*Beta-blockers interfere with potassium transport into cells and can potentially cause hyperkalaemia in renal failure patients - remember beta-agonists, e.g. Salbutamol, are sometimes used as emergency treatment.

**Both unfractionated and low-molecular weight heparin can cause hyperkalaemia. This is thought to be caused by inhibition of aldosterone secretion.