Forensic Serology Questions and Answers

Answer: A

Sodium fluoride, used to preserve blood samples, inhibits the enzyme enolase, option (A), which is in the glycolytic pathway of the red cells, and so stops the breakdown of glucose and the microbial formation of alcohol in the sample. Esterase, peptidase and phosphokinase are not the enzymes it blocks for this use, so (B), (C) and (D) are wrong. This is why sodium fluoride is the preservative for blood alcohol samples.

Answer: A

The three genetic loci that control phosphoglucomutase (PGM) polymorphism are locus 1 on chromosome 1, locus 2 on chromosome 4 and locus 3 on chromosome 6, option (A). PGM1 is the locus whose variants are used in blood group typing of dried stains in forensic work. Chromosomes 8, 10 and 14 do not carry a PGM locus, so (B), (C) and (D) are wrong. The PGM system was widely used for typing dried blood stains in the 1980s before DNA methods came in, and it has subtypes of the PGM1 enzyme that can be separated by isoelectric focusing.

Answer: B

The technique used for increasing the amount of a specific segment of DNA is DNA amplification, option (B), which is done by the polymerase chain reaction, repeated cycles of heating, annealing the primers and extending, so that the segment is copied exponentially. DNA replication is the natural copying of the whole genome in the cell, denaturation is the separation of the two strands, which is one step of the cycle, and VNTR is a type of repeat region used in typing, so (A), (C) and (D) are wrong.

Answer: A

Lectins are plant proteins, option (A), which bind to specific carbohydrates and are found in seeds such as the castor bean, and they are used in the typing of blood groups in forensic work. Actin is a protein of the muscle in animals, and mycin, as in streptomycin, is an antibiotic from microbes and not a plant protein. So (B) and (C) are wrong, and (D) is wrong because only one of the three is a plant protein.

Answer: A

The D-loop is a part of mitochondrial DNA, option (A), the displacement loop, a non-coding control region that carries the origin of replication of the heavy strand and the hypervariable segments used in forensic typing of old bones and hair. Y-loop, S-loop and T-loop are not parts of the mitochondrial genome, although a T-loop is seen at the ends of chromosomes, so (B), (C) and (D) are wrong. Because mitochondrial DNA is inherited from the mother and present in many copies, it is useful when nuclear DNA is too degraded to be typed.

Answer: D

HLA is not a red cell blood group system, option (D), since the human leukocyte antigens are on the white cells and most other nucleated cells and are used in tissue typing for transplants. The Kell, Duffy and ABO systems are antigens on the surface of the red cells and are recognised blood group systems, so (A), (B) and (C) are red cell systems. Other red cell systems include the Rh, MNS, Lewis and Kidd systems, which are also used in tests of parentage and in the examination of dried blood.

Answer: D

A forensic scientist who examines dried blood is expected to answer all three questions, option (D): is it blood, is it human blood, and which group does it belong to. The first is answered by presumptive and confirmatory tests, the second by the precipitin test, and the third by grouping and, in current work, DNA typing. Since each of (A), (B) and (C) is only one of the questions, the answer is all of the above. Each test narrows the question: a negative result in the first one ends the examination, and a positive result in the third helps to link the stain with a person.

Answer: C

The test is Lugol's iodine test.

For a sexual assault examination the swab is smeared on a slide, stained with Lugol's iodine, and examined under the microscope for the brown-stained flat cells.

Epithelial cells of the vagina are rich in glycogen, which stains mahogany brown with Lugol's iodine, so the cells can be recognised in a swab in a case of sexual assault.

Barberio's and Florence's tests are chemical tests for semen, using picric acid and iodine-potassium iodide, and the precipitin test identifies the species of origin of blood or other proteins. They do not identify vaginal epithelial cells.

Answer: C

Both statements are correct: Both (A) and (R) are correct.

In the absorption-inhibition method a known antiserum is allowed to absorb the antigen being tested, by placing the antigenic material in contact with its homologous antibody (A). If the antigen matches, the antibody is used up, so the serum becomes less potent, and the loss of its activity is measured (R).

The options that call either statement incorrect are therefore wrong.

Answer: B

The reagent is periodic acid-Schiff reagent.

The test is used when the question is whether a stain or swab is of vaginal origin, as in cases in which sexual offences are alleged.

Vaginal epithelial cells hold glycogen, and the PAS stain oxidises the sugar with periodic acid and then gives a magenta colour with Schiff reagent, so glycogenated cells are detected.

Benzidine reagent is used for blood, Nessler's reagent for ammonia, and iodoplatinate for alkaloids and drugs. None of them detects glycogen.