MLT GlobeForensic Science: Forensic serology & body fluids – page 2
40 Forensic Science MCQs on Forensic serology & body fluids with answers and explanations.
The Brentamine Fast Blue test with alpha-naphthyl phosphate screens for semen by detecting which enzyme?
Seminal acid phosphatase hydrolyses alpha-naphthyl phosphate. The released naphthol couples with Fast Blue B dye to give a purple colour within seconds.
Why is the acid phosphatase test presumptive for semen?
Seminal fluid has very high acid phosphatase, but lower levels occur in vaginal fluid and some plants. Timing and intensity help, but confirmation is required.
Why is PSA testing valuable when semen is suspected but no sperm are seen?
Azoospermic or vasectomised men produce seminal fluid lacking sperm. PSA still identifies seminal fluid in such samples.
What is the principle of the Phadebas test for saliva?
Phadebas uses insoluble starch linked to a blue dye. Amylase digestion releases soluble blue dye, which can be seen or measured, or used as a paper press test.
In a starch-iodine radial diffusion assay, what indicates amylase activity from a stain extract?
Iodine stains intact starch blue-black. Amylase diffusing from the well digests the starch, leaving a clear zone whose size reflects enzyme activity.
In the urease test for a suspected urine stain, what is actually detected?
Urease hydrolyses urea to ammonia and carbon dioxide. The ammonia raises pH, detected by an indicator, suggesting urine.
Lugol's iodine is used when examining a suspected vaginal secretion stain. What does it highlight?
Vaginal squamous cells are rich in glycogen, which stains dark brown with iodine. This is indicative, not specific, as other epithelia may contain glycogen.
How is the impact angle of a blood drop estimated from an elliptical stain?
The more acute the impact, the more elongated the stain. Arcsine of width/length gives the impact angle.
Which bloodstain pattern results from blood released from a moving object during a swing?
Blood thrown from an object in motion forms a linear or curvilinear trail of stains on nearby surfaces, called cast-off.
Small bloodstains contain air bubbles or vacuoles and appear diluted. Which mechanism is most likely?
Blood expelled by breath mixes with air and saliva or mucus, producing bubble rings and paler stains. Small stains may resemble impact spatter.
Why does a dried bloodstain change from red to brown and then black over time?
Oxyhaemoglobin oxidises to methaemoglobin and then haemichrome, darkening the stain. The rate depends on temperature, humidity and substrate, so dating by colour is imprecise.
Which technique was classically used to determine the ABO group from a dried bloodstain?
Antisera bind antigens on the stain fibres, unbound antibody is washed off, and bound antibody is eluted by heat and detected with indicator red cells.
An immunochromatographic human haemoglobin test is negative on a heavy, fresh bloodstain. Which explanation is most likely?
Excess antigen saturates both capture and detection antibodies so the sandwich complex fails to form. Retesting a diluted extract resolves this false negative.
In Ouchterlony double diffusion, precipitin lines from two adjacent wells join smoothly. What does this indicate?
A smooth fusion (line of identity) shows the antigens share the same epitopes. Crossing lines indicate non-identity; a spur indicates partial identity.
Semenogelin, a target of some confirmatory semen tests, is mainly secreted by which structure?
Semenogelin is the major seminal vesicle protein forming the ejaculate coagulum. Lateral-flow assays for it complement PSA testing.
What is the main advantage of an antibody-based lateral-flow test (e.g., RSID-Saliva) over Phadebas?
Enzyme-activity tests respond to pancreatic, bacterial and non-human amylases. Monoclonal antibodies against human salivary alpha-amylase improve specificity.
Edelman's test, used for faecal material, gives green fluorescence with zinc salts. Which substance does it detect?
Urobilin, a bile pigment breakdown product in faeces, forms a zinc complex that fluoresces green under UV light, supporting faecal origin.
Stain long axes are projected back to find their area of convergence. What additional information is needed to estimate the area of origin in three dimensions?
Convergence is a two-dimensional location. Using each stain's impact angle allows projection upward from the surface to estimate the height of the blood source.
Approximately what proportion of people are 'secretors' who secrete ABH blood group antigens into body fluids?
Secretor status depends on a functional FUT2 gene. About 80% of people are secretors, allowing ABO typing of saliva or semen before DNA methods.
Which finding in a vaginal bloodstain most supports menstrual blood rather than peripheral blood?
Menstrual blood contains large amounts of fibrin degradation products due to fibrinolysis in the endometrium. Messenger RNA markers such as MMPs are also used.