MLT GlobeForensic Science: Forensic serology & body fluids
40 Forensic Science MCQs on Forensic serology & body fluids with answers and explanations.
What colour change indicates a positive Kastle-Meyer test for blood?
Haem's peroxidase-like activity uses hydrogen peroxide to oxidise colourless reduced phenolphthalein (phenolphthalin) to phenolphthalein, which is pink in the alkaline reagent.
Luminol is especially useful at scenes for what purpose?
Luminol is oxidised in the presence of haem to emit blue light visible in darkness, revealing very dilute or cleaned blood. It is presumptive only.
The Takayama test for blood produces which crystals?
In Takayama's test, pyridine and a reducing sugar in alkaline solution form pyridine ferroprotoporphyrin (haemochromogen) crystals, a microcrystalline confirmatory test for haem.
The precipitin test for species of origin is based on which principle?
Stain extract (antigen) meets antiserum raised against a species' proteins. A visible precipitin line or ring shows that species' proteins are present.
A tetramethylbenzidine (TMB) presumptive blood test turns which colour when positive?
TMB is oxidised through haem's peroxidase-like activity to a blue-green product. It replaced benzidine, which is carcinogenic.
Prostate-specific antigen (p30), used to identify semen, is produced mainly by which organ?
PSA, also called p30 for its approximate 30 kDa mass, is a serine protease secreted by prostatic epithelium into seminal plasma.
In Christmas tree staining of a semen smear, how do sperm appear?
Nuclear fast red stains sperm heads red, and picroindigocarmine stains tails and cytoplasm green, making sperm easy to spot among epithelial cells.
How do dried semen stains typically appear when examined with an alternate light source or UV light?
Flavins and other fluorophores in seminal fluid fluoresce under UV or blue light, helping locate stains, although many other substances also fluoresce.
Which enzyme activity is the basis of most presumptive tests for saliva?
Saliva contains high alpha-amylase activity. Detecting starch hydrolysis indicates possible saliva, but amylase occurs in other fluids too.
The Jaffe reaction, used to help identify urine stains, detects which substance?
Creatinine reacts with alkaline picrate to form an orange-red complex. High creatinine concentration supports the presence of urine.
A single drop of blood falls vertically onto a smooth, flat, horizontal surface. What stain shape is expected?
At a 90° impact angle, the drop spreads evenly in all directions, forming a near-circular stain. Rough surfaces may add scalloped edges or satellites.
In an elongated bloodstain, what indicates the direction in which the drop was travelling?
As the drop strikes at an angle, blood moves forward and forms a tail or satellite spines on the leading edge, pointing in the direction of travel.
A bloody shoe leaves a recognizable sole impression on a floor. How is this stain classified?
A transfer stain arises when a bloody object contacts a surface; recognisable features can reveal the object's shape, such as a shoe sole.
Which finding in a vaginal swab extract confirms the presence of semen?
Seeing sperm cells is confirmatory for semen. Colour tests and fluorescence are presumptive because other materials can react.
Why is the Kastle-Meyer test considered presumptive rather than confirmatory for blood?
Vegetable peroxidases (e.g., horseradish, potato) and chemical oxidants such as copper salts can produce the colour, so a positive result indicates only possible blood.
In the Kastle-Meyer test, colour appears after adding the phenolphthalin reagent but before adding hydrogen peroxide. How is this interpreted?
Blood needs peroxide to develop colour. Colour before peroxide addition suggests an oxidising substance, so the result cannot be taken as indicating blood.
A floor gives a strong, brief luminol reaction after cleaning. Which substance is a common cause of such a false positive?
Hypochlorite oxidises luminol directly, producing light without haem. Its reaction is typically bright and fades quickly compared with blood.
The Teichmann test for blood involves heating a stain with glacial acetic acid and a halide. What crystals form?
Teichmann's test converts haem to haemin (ferriprotoporphyrin chloride), forming brown rhombic crystals. It is less sensitive than Takayama and affected by overheating.
Which modern method is commonly used to confirm that a bloodstain is of human origin?
Lateral-flow cassettes use monoclonal antibodies against human haemoglobin (or glycophorin A). They are fast, sensitive and species-specific, though higher primates may cross-react.
What colour indicates a positive leucomalachite green (LMG) test for blood?
Colourless leucomalachite green is oxidised to malachite green in the presence of haem and peroxide, producing a green colour.