A rapid finger-prick blood test to help diagnosis deep vein thrombosis is developed. Comparing the test to current standard techniques a study is done on 1,000 patients:
What is the specificity of the new test?
Correct Answer C: Specificity = true negatives / (true negatives + false positives) = 680 / (680 + 100)
Screening test statistics: It would be unusual for a medical exam not to feature a question based around screening test statistics. The available data should be used to construct a contingency table as below:
TP = true positive; FP = false positive; TN = true negative; FN = false negative
The table below lists the main statistical terms used in relation to screening tests:
Positive and negative predictive values are prevalence dependent. Likelihood ratios are not prevalence dependent.
Which one of the following would shift the oxygen dissociation curve to the left?
Correct Answer A: Oxygen dissociation curve:
Oxygen dissociation curve: he oxygen dissociation curve describes the relationship between the percentage of saturated haemoglobin and partial pressure of oxygen in the blood. It is not affected by haemoglobin concentration.
Basics:
*2,3-diphosphoglycerate
A 22-year-old male with a history of familial adenomatous polyposis (FAP) has a total colectomy.
What is the mode of inheritance of FAP?
Correct Answer D: Colorectal cancer: genetics:
It is currently thought there are three types of colon cancer:
Studies have shown that sporadic colon cancer may be due to a series of genetic mutations. For example, more than half of colon cancers show allelic loss of the APC gene. It is believed a further series of gene abnormalities e.g. activation of the K-ras oncogene, deletion of p53 and DCC tumour suppressor genes lead to invasive carcinoma.
HNPCC, an autosomal dominant condition, is the most common form of inherited colon cancer.
Around 90% of patients develop cancers, often of the proximal colon, which are usually poorly differentiated and highly aggressive. Currently seven mutations have been identified, which affect genes involved in DNA mismatch repair leading to microsatellite instability.
The most common genes involved are:
The Amsterdam criteria are sometimes used to aid diagnosis:
FAP is a rare autosomal dominant condition which leads to the formation of hundreds of polyps by the age of 30- 40 years. Patients inevitably develop carcinoma. It is due to a mutation in a tumour suppressor gene called adenomatous polyposis coli gene (APC), located on chromosome 5. Genetic testing can be done by analyzing DNA from a patients white blood cells. Patients generally have a total colectomy with ileo-anal pouch formation in their twenties.
Patients with FAP are also at risk from duodenal tumours. A variant of FAP called Gardner's syndrome can also feature osteomas of the skull and mandible, retinal pigmentation, thyroid carcinoma and epidermoid cysts on the skin.
Which one of the following diseases is most strongly associated with HLA antigen DR4?
Correct Answer D: Rheumatoid arthritis - HLA DR4.
Around 70% of patients with rheumatoid arthritis are HLA-DR4. Patients with Felty's syndrome (a triad of rheumatoid arthritis, splenomegaly and neutropaenia) are even more strongly associated with 90% being HLADR4.
HLA associations: HLA antigens are encoded for by genes on chromosome 6. HLA A, B and C are class I antigens whilst DP, DQ, DR are class II antigens. Questions are often based around which diseases have strong HLA associations.
The most important associations are listed below:
HLA-A3
HLA-B5
HLA-B27 A
HLA-DQ2/DQ8
HLA-DR2
HLA-DR3
HLA-DR4
*type 1 diabetes mellitus is associated with HLA-DR3 but is more strongly associated with HLA-DR4.
Which one of the following causes of primary immunodeficiency is a T-cell disorder?
Correct Answer D: DiGeorge syndrome - a T-cell disorder.
DiGeorge syndrome is a primary immunodeficiency disorder caused by T-cell deficiency and dysfunction. It is an example of a microdeletion syndrome. Patients are consequently at increased risk of viral and fungal infections.
Primary immunodeficiency disorders may be classified according to which component of the immune system they affect:
1- Neutrophil disorders:
2- B-cell disorders:
3- T-cell disorders:
4- Combined B- and T-cell disorders:
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