I debated putting up the answers. I’m putting the answers up now for one reason. I’ve noticed that another individual has unfortunately pirated nearly the entire content of my site (it’s equal parts amusing and abhorrent to behold). None of this harms anyone, but this individual has also misguidedly attempted to answer the questions. The following are the correct answers based on my own limited knowledge and input from my colleagues. Please feel free to include your own inputs.
1) B is the correct answer choice. This concept will be on your exam. Know it well because these are guaranteed points. Sensitivity refers to the percentage of true positives who are correctly identified as being positive. Specificity refers to the percentage of true negatives who are correctly identified as negative. In theory, you’d want a test to be 100% sensitive and 100% specific. In reality, this is very difficult to achieve. For example, an HIV test that is extremely sensitive (near 100%) will identify every individual who really has HIV but it may also falsely identify many people as having HIV when they don’t really have it (the sensitivity is high but specificity is low). On the other hand, a very specific test will not falsely identify anyone as having HIV but it will also fail to identify some who do have it.
So for this question, you can clearly see that Procedure 2 is more sensitive than Procedure 1 – this eliminates choices A & C. You can also see that Procedure 1 is more specific than Procedure 2 so you can eliminate choice D. You are left with choice B which is clearly true.
2) A is the correct answer. The equation is the formula for calculating sensitivity. Sensitivity = Number of True Positives/(Number of True Positives + Number of False Negatives). It is important that you differentiate sensitivity from specificity. Specificity = Number of True Negatives/(Number of True Negatives + Number of False Positives)
3) The correct answer is A. The presence of WBCs and nitrites in urine is a classical indication of a bacterial infection of the urinary tract (pyelonephritis). The nitrites are a result of gram negative bacteria reducing urinary nitrates to nitrites. The presence of WBCs is due to a bacterial infection attracting a neutrophilic response from the body. The answer is NOT “glomerulonephritis” b/c that can be a result of a whole variety of different causes, including diabetes, SLE, drugs or pathogens.
4) The correct answer is B. In healthy folks, the kidneys prevent albumin and other proteins from entering the urine as waste. If the kidneys are damaged however, it will allow proteins to pass into the urine. The first type of protein to appear in urine is albumin as its molecular size is smaller than most other proteins.
5) The correct answer is C. Cortisol counteracts insulin. It will increase levels of glucose in blood (Ilyanok, N).
6) The correct answer is C. The front type suggests that the patient is type AB but the back type suggests he is type B. This is considered an ABO discrepancy. The explanation is that this person is most likely A2B and thus producing anti-A1. Approximately 20% of people who are type A are type A2 while 80% are type A1. (Epps-Clarke, L)
7) The correct answer is B. There isn’t much to remember about Leptospira for this exam so just try to remember that Fletcher’s media is used to culture it.
8) The correct answer is B. A key identifying characteristic for Eikenella corrodens is the production of a bleach-like odor. It is a gram negative bacteria that also causes pitting on agar plates.
9) The correct answer is A. The presence of rheumatoid factor in the blood may result in false positive results with the VDRL test. The VDRL test is a nonspecific serological screening test for syphilis – a variety of factors may cause it to give you a false positive result. These include rheumatic fever, some drugs, pregnancy and leprosy. On the other hand, FT-ABS is a test specific for treponema pallidum as it detects antibodies against T. pallidum.
10) The correct answer is C. Since TSH, T3 and T4 are all elevated, this suggests a pituitary tumor. The tumor is causing production of excess TSH, which is in turn causing elevated production of T3 and T4. Answer choice A is wrong because hypothyroidism would present with low T3 & T4 and high TSH. Answer choice B is wrong as hyperthyroidism would present with high T3 & T4 and low TSH.
11) The correct answer is A, because PTH causes an elevation of calcium in the blood. This happens in several ways; including bone resorption (bone breakdown which releases calcium into blood), and increased calcium uptake by the kidneys and intestines (so less is lost by your body). Try to understand the following diagram concerning calcium homeostasis as it’s likely to account for 1 or more questions on your exam.
12) The correct answer is D. To answer this question you’ll use process of elimination and some knowledge you would have gained while training in a micro lab. Since a yellow color is produced on the slant aspect of the TSI tube, the organism can ferment lactose or sucrose. Keeping in mind that this is a lactose fermenter, take a look at the Enterobacteriaceae chart (in the micro section) and immediately rule out everything on the right-hand side of the chart (like Salmonella) since the organism in question is a LACTOSE FERMENTER. Next, recognize that a mucoid appearance is due to the presence of a capsule. If you have rotated in a microbiology lab by now, you may recall that klebsiella has a mucoid appearance while E. coli produces dry colonies – rule out E. coli. You are down to only choices C & D. Which one of these two mucoid, lactose fermenters is the organism in question? The only way to differentiate them is via the indole test. Klebsiella oxytoca is indole positive while Klebsiella pneumoniae is indole negative. The answer is Klebsiella oxytoca.
By the way, this is a tricky question. You may have been tempted to rule out Klebsiella earlier because all Klebsiella (other than Klebsiella oxytoca) are indole negative while E. coli are indole positive. However, recognize that E. coli colonies appear dry while Klebsiella colonies appear mucoid on agar. You will see this when you train in a micro lab.
13) The correct answer is A. Follow the chart for gram negative bacilli. What organism is PAD + and Indole +? That would be Proteus vulgaris. For practical purposes (and by extension for the exam), it’s important to know that P. vulgaris is Indole positive and P. mirabilis is Indole negative. This was on the exam because it is practical knowledge.
14) The correct answer is A. If you only know a few things about helicobacter pylori, you should know this: It is curved, can infect the GI tract, and the urease it produces may cause ulcers.
15) The more likely answer is B. In this case, there is clearly a major problem with the controls. As you probably know, enzymes are very sensitive to temperature. While most enzymes work more rapidly at higher temperatures, they will also be denatured by temperatures far beyond their acceptable range, losing much of their functionality. Choice A is incorrect because the control wouldn’t start giving results this erroneous (being off by 3 standard deviations is pretty bad!) just because it recently expired.
16) The correct answer is D. You are PRETTY SURE that the person has EITHER HIV-1 or HIV-2 b/c the combo ELISA test tells you so. This test is a cheap, catch-all test so it’s run first. However, this test can give you false positives and you don’t want to go around telling someone they have this disease unless you’re sure they have it. Thus, a more expensive, time consuming and sure confirmation for the ELISA test is a Western Blot. This test is specific so it can differentiate btwn HIV-1 and HIV-2. Since the Western Blot for HIV-1 came back as inconclusive, there’s probably a good chance that the ELISA test originally detected HIV-2. Thus you would run a Western Blot for HIV-2 in order to confirm.
17) The correct answer is C. PCR is an artificial process generating multiple copies of a particular DNA sequence. The first step involves denaturation of the bonds between the two complementary strands so they separate (unzip) from each other; Next, short DNA primers attach (anneal) to one of the separated DNA strands; Finally, DNA Polymerase extends the DNA along the primer, transcribing a new strand of DNA based on the mirror image strand opposite the primer.
18) The correct answer is A. RAST is a blood test used to determine specific IgE antibodies to known antigens.
19) The correct answer is C. There may be several reasons for a high hematocrit – including the obvious possibility that this is a newborn who demonstrate a normal hematocrit range of 55to 68%. Another likely reason for a high hematocrit is dehydration – the individual just happens to have less fluid in the body. Less frequent causes include Polycythemia Vera or high hematocrit due to blood doping, a favorite of several infamous athletes including Lance Armstrong. Note that blood doping with EPO is a very bad idea because the increased viscosity may very well lead to a heart attack or stroke as you’re receiving the First Place Gold Medal! Answer choice A is wrong for two reasons: (1) you use EDTA rather than sodium citrate for CBC blood collection and (2) blood collection is a standardized practice and thus your goal in testing the hematocrit is to determine the actual hematocrit – not to artificially disturb the ratio of anticoagulant in order to arrive at a normalized determination of hematocrit! Answer choice B is wrong because you don’t collect CBC specimens in heparin – you use EDTA (Natallia Ilyanok).
20) The correct answer is C – the exciting light is the dangerous, high energy light produced by the machine. Answer choice B refers to the lower energy light produced when excited electrons (previously excited to higher energy levels by the machine) fall back to lower energy levels. The cover light is just regular light helping you see what’s happening.
21) The correct answer is C. Blood is sometimes frozen in order to preserve rare types. Glycerol is used in the freezing process. Once frozen, the expiration date of the blood is extended to 10 years from the date of phlebotomy. If the blood is needed, it is defrosted and the new expiration time becomes only 24 hours from the time it is defrosted (Polina Gurevich).
As an aside, we recently had a patient with anti-U antibody. We needed to give this person blood from a donor who is negative for U antigen. Unfortunately, only 0.01% of the population is negative for U antigen. We were able to find this blood in frozen/glycerolized form at the New York Blood Center. We had to have a good idea of what time the patient would need to receive the blood though. That’s b/c once defrosted, the blood is only good for 24 hours before it expires.
22) The correct answer choice is B. To determine if this patient has been re-infected you would have to perform a VDRL test. Answer choice A is incorrect because a TPA test may remain active for the life of the patient so it is not useful in determining reinfection or treatment.
23) D is the correct answer choice. Use the process of elimination. I strongly urge you try to understand the reasoning here because it is a perfect example of a secondary level question. As a secondary level question you will first make one determination and then use that determination to come to a final conclusion. Reread this question carefully at the end to be sure you’re actually answering the question. First, the question tells you that the patient has Jka antigen on his cells – you don’t even need to worry about this because none of the answer choices address Jka! Next, we rule out the possibility of K antibody because testing of the patients serum against cells positive for the K antigen demonstrated a reaction strength of zero; thus the patient is not producing anti-K antibody – However, the question is not asking you to determine what antibody the patient is producing! It’s asking you to go further and determine the antigenic makeup of the patients red cells – the fact that the patient is not producing anti-K antibody could mean either of two things with regard to whether the patient has K antigen on his cells: (1) the patient is antigenically positive for K so that’s why he’s not producing antibody against his own cells or (2) the patient is antigenically negative for K but has never been exposed to K antigen through a foreign blood transfusion so his body has never been prompted into producing anti-K antibody. So you essentially cannot confirm the presence or absence of K antigen on the patients red cells. Finally, the patients serum reacted strongly against cells with c antigen on them and thus he is definitely producing anti-c antibody – if he produces anti-c antibody then we can rule out the presence of c antigen on his red cells because he would not produce an antibody against his own cells!
24) The correct answer choice is B. Unlike the urine protein chemistry dipstick (Reagent strip) which only detects albumin, the SSA test detects albumin as well as other proteins like Bence-Jones proteins.
25) This question cannot be answered because I don’t have a picture of the immature granulocyte presented. The picture of the granulocyte itself was the clue to which type of stain ought to be used. I only included the wording of this question to show you the types of questions you may encounter. You may already be aware that special leukemia stains are used to help distinguish one type of cell from another. For example, leukocyte specific esterase identifies granulocytes (which show red granules) while leukocyte nonspecific esterase identifies monocytes and megakaryocytes (which show black granules).
26) The correct answer is E – deficient sodium (aka hyponatremia). SIADH is a disease characterized by release of excess ADH (antidiuretic hormone) from the posterior pituitary. Anti (against) diuresis (urine production) means that you won’t be urinating as much so body fluid levels increase. This will dilute the concentration of sodium in your body.
27) The correct answer is B. Fiber strands (commonly fibers from diapers) resemble hyaline casts and may be mistaken for such.
28) The correct answer is B. Troponins are contractile proteins that regulate muscle contraction along with tropomyosin and calcium. Recall that troponin levels in blood may be used to determine if someone has suffered a cardiac injury.
29) The correct answer is A – Hba1c levels cannot be used to monitor blood glucose levels in patients with sickle cell disease. As you may know, sugars tend to attach to RBCs over time. The HBa1c test measures this degree of glycation of RBCs (if you have more sugars in blood you’ll have more sugar attaching to the RBCs over time). Any disorder that causes RBCs to die prematurely (sickle cell disease, G6PD deficiency, etc) will result in an underestimation of the Hba1c levels b/c the old, glycated cells deformed and died, being replaced by newer cells which are not very glycated. Thus, the test is not valid in patients with sickle cell disease.