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MT MLO practice questions

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Question 1Pre-analytical considerations: specimen collection, handling, and rejection criteria

A blood specimen for a serum chemistry panel is collected in a plain tube with no additive or clot activator. What is the primary pre-analytical concern with this tube type?

  1. Platelets will release potassium for a shorter period than in a clot-activator tube
  2. Complete clotting may be delayed, risking fibrin formation in the separated serum Correct answer.
  3. Serum glucose will be consumed by glycolysis unless fluoride is present
  4. The absence of anticoagulant causes immediate hemolysis of red cells

Explanation

Plain glass or plastic tubes without clot activator rely on contact activation and can take considerably longer to clot completely. If serum is separated before clotting is complete, fibrin strands can form in the sample and interfere with analysis. This is why many laboratories require adequate clotting time before centrifugation.

Question 2Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which collection practice best minimizes tissue-fluid contamination of a venous blood specimen?

  1. Releasing the tourniquet before the last tube is filled
  2. Using a needle that is too short to reach the vein lumen
  3. Allowing the first few drops of blood to flow freely before attaching the tube Correct answer.
  4. Drawing with the tourniquet applied for several minutes before puncture

Explanation

Tissue fluid and small clots can enter the sample at the start of the draw, so discarding the initial drops and letting blood flow freely reduces contamination. Prolonged tourniquet time instead concentrates cells and proteins. Short needles or premature tourniquet release create other pre-analytical problems rather than preventing tissue-fluid admixture.

Question 3Pre-analytical considerations: specimen collection, handling, and rejection criteria

A urine specimen for routine culture is left at room temperature on the ward for six hours before transport. What is the most likely pre-analytical consequence?

  1. False reduction in leukocyte esterase activity
  2. Loss of bacterial viability leading to a sterile culture
  3. Increased specific gravity from evaporation
  4. Overgrowth of contaminating organisms that obscures the true pathogen Correct answer.

Explanation

Urine supports bacterial multiplication, so delay at room temperature allows commensal and contaminant organisms to multiply and can mask the actual pathogen. Standard practice is prompt transport or refrigeration, or use of a preservative tube. Viability generally increases rather than disappears, and evaporation in a closed container is negligible.

Question 4Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which specimen is most clearly unacceptable for routine coagulation testing on a citrated tube?

  1. A tube filled to 90 percent of the indicated draw volume
  2. A tube centrifuged within one hour of collection
  3. A tube with visible clots despite adequate fill volume Correct answer.
  4. A tube transported at ambient temperature for thirty minutes

Explanation

Visible clots indicate that fibrin has formed, meaning the anticoagulant did not preserve the sample; such specimens must be rejected for coagulation testing. Underfilled tubes are also problematic, but 90 percent fill is often within many laboratories' acceptance limits. Clotting is an absolute rejection criterion regardless of fill volume.

Question 5Pre-analytical considerations: specimen collection, handling, and rejection criteria

During a draw from an arm with a running intravenous line, which approach is preferred for obtaining a blood specimen?

  1. Draw from the same arm above the infusion site after flushing the line
  2. Draw from the same arm below the infusion site without stopping the infusion
  3. Draw from the infusion tubing itself using a syringe
  4. Draw from the opposite arm whenever possible, or stop the infusion and follow line-draw protocol Correct answer.

Explanation

Blood drawn distal to or through an infusion line is diluted and contaminated by the infusate, so the opposite arm is preferred. If no alternative site exists, the infusion must be paused and a validated line-draw procedure followed. Drawing from the tubing directly yields infusate, not a representative blood sample.

Question 6Pre-analytical considerations: specimen collection, handling, and rejection criteria

A sputum specimen for bacterial culture is described as consisting mostly of saliva. How should the laboratory respond?

  1. Reject it as inadequate and request a properly collected specimen Correct answer.
  2. Accept it and report normal flora
  3. Accept it only if the patient has a fever
  4. Process it but dilute it to reduce oral contamination

Explanation

Saliva is not representative of lower respiratory secretions and yields misleading culture results, so such specimens should be rejected with a request for a proper collection. Reporting normal flora from saliva provides no diagnostic value. Dilution does not correct the fundamental sampling error.

Question 7Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which action during blood collection is most likely to cause in vitro hemolysis?

  1. Pulling the syringe plunger back forcefully Correct answer.
  2. Inverting tubes gently after collection
  3. Using a 21-gauge needle for a routine adult draw
  4. Allowing the alcohol to dry before puncture

Explanation

Forcible aspiration with a syringe creates shear forces and turbulence that rupture red cells, releasing hemoglobin and intracellular analytes. Letting the antiseptic dry, gentle inversion, and a moderate needle gauge are all recommended practices that reduce hemolysis. The key is to avoid mechanical trauma during collection.

Question 8Pre-analytical considerations: specimen collection, handling, and rejection criteria

What is the recommended maximum time before separating serum or plasma from cells for most routine chemistry analytes when the sample is kept at room temperature?

  1. Approximately two hours Correct answer.
  2. Up to twenty-four hours
  3. Immediately, always within minutes of collection
  4. Approximately twelve hours

Explanation

Many chemistry analytes change when left in contact with cells, so serum or plasma is generally separated within about two hours of collection at room temperature. Immediate separation is not always feasible, and delays of twelve to twenty-four hours allow significant changes in glucose, potassium, and other analytes.

Question 9Pre-analytical considerations: specimen collection, handling, and rejection criteria

A patient's potassium result is markedly higher than a previous value, and the specimen is noted to be hemolyzed. What is the most appropriate interpretation?

  1. Hemolysis has no effect on potassium and the result is valid
  2. The result reflects a true hyperkalemia and should be reported without comment
  3. The result is falsely low because hemoglobin interferes with the assay
  4. The result may be falsely elevated by red cell release and should be evaluated or recollected Correct answer.

Explanation

Red cells contain potassium at much higher concentration than plasma, so hemolysis releases potassium and falsely elevates the measured value. The laboratory should flag the interference and, where clinically important, request a repeat specimen. Reporting the value as a true result risks an incorrect clinical decision.

Question 10Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which order of draw is correct when collecting a serum tube, a sodium citrate coagulation tube, and an EDTA tube during a single venipuncture?

  1. Citrate, EDTA, serum
  2. Serum, citrate, EDTA
  3. Citrate, serum, EDTA Correct answer.
  4. EDTA, citrate, serum

Explanation

Coagulation tubes must be drawn before tubes containing other additives to avoid carryover of anticoagulants that would interfere with clotting tests. Serum tubes follow, and EDTA is drawn later. This sequence protects the most interference-sensitive test from contamination.

Question 11Pre-analytical considerations: specimen collection, handling, and rejection criteria

A blood culture set is ordered for an adult. Which collection practice is most consistent with current recommendations?

  1. Collect the culture after antibiotics have been started to assess response
  2. Collect two sets from two separate venipuncture sites Correct answer.
  3. Collect a single aerobic bottle to reduce patient discomfort
  4. Collect both bottles of a set from the same syringe without regard to volume

Explanation

Multiple sets from separate sites improve detection of bacteremia and help distinguish contamination from true infection. A single bottle reduces sensitivity, and timing relative to antibiotics matters because prior antimicrobial therapy lowers yield. Volume per bottle also affects recovery, so each bottle should be filled appropriately.

Question 12Pre-analytical considerations: specimen collection, handling, and rejection criteria

A 24-hour urine collection is submitted with a recorded volume that appears incomplete, and the container was not refrigerated or preserved. Which action is most appropriate?

  1. Reject the collection and request a repeat with proper instructions Correct answer.
  2. Report results with a comment about possible underestimation
  3. Accept it if the creatinine concentration is within reference range
  4. Multiply the result by an estimated completion factor

Explanation

An incomplete or improperly preserved timed collection cannot yield reliable quantitative results, so rejection and recollection with clear instructions is appropriate. Estimating or correcting for incompleteness introduces unverifiable error. A normal creatinine concentration does not confirm completeness.

Question 13Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which situation most warrants rejection of a citrated coagulation tube?

  1. The tube was filled to the marked line and mixed by gentle inversion
  2. The tube was centrifuged at the laboratory's validated settings
  3. The tube's hematocrit exceeds the level at which the standard fill volume is inadequate Correct answer.
  4. The tube was drawn from a line and labeled with the line-draw protocol

Explanation

When hematocrit is very high, the plasma proportion of the sample is reduced, so the standard citrate volume over-anticoagulates the specimen and can prolong clotting times. Laboratories must adjust fill volume or reject and recollect per protocol. Proper fill, validated centrifugation, and documented line draws are acceptable.

Question 14Pre-analytical considerations: specimen collection, handling, and rejection criteria

A specimen for arterial blood gas analysis is collected in a plastic syringe and transported without ice to the laboratory, arriving thirty minutes after collection. What is the main pre-analytical concern?

  1. Absence of ice causes the sample to clot despite heparin
  2. The sample will show falsely increased oxygen due to exposure to air
  3. Ongoing cellular metabolism alters pH and gas tensions Correct answer.
  4. The plastic syringe causes immediate loss of carbon dioxide through the wall

Explanation

Cells in the sample continue to consume oxygen and produce carbon dioxide and acid, shifting pH and gas values over time. Prompt analysis or appropriate cooling slows this metabolism. Clotting relates to heparin mixing rather than temperature, and air exposure is a separate issue from transport temperature.

Question 15Pre-analytical considerations: specimen collection, handling, and rejection criteria

A stool specimen for ova and parasite examination is received as a single specimen collected three days earlier and stored at room temperature. What is the most appropriate action?

  1. Accept it because formalin fixation occurs in the laboratory
  2. Reject it because parasite morphology and viability may be compromised Correct answer.
  3. Accept it if the patient has no symptoms
  4. Reject it only if it is liquid in consistency

Explanation

Delays and warm storage allow protozoan trophozoites to degenerate and alter parasite morphology, reducing detection. Specimens should be preserved promptly or submitted within the laboratory's defined time limit. Consistency affects which parasites are likely but does not make a three-day-old unpreserved specimen acceptable.

Question 16Pre-analytical considerations: specimen collection, handling, and rejection criteria

A phlebotomist labels a tube at the bedside before entering the patient's room, using a label printed earlier. What is the principal risk?

  1. The specimen may be mislabeled and attributed to the wrong patient Correct answer.
  2. The tube may be underfilled because labeling takes time
  3. The tube may clot because labeling delays inversion
  4. The additive may degrade while the label is applied

Explanation

Labels must be applied after collection and in the presence of the patient, with identity confirmed at that time. Pre-labeling removes the verification step and creates a serious risk of attributing a specimen to the wrong patient. Labeling does not meaningfully affect fill volume or additive stability.

Question 17Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which practice is correct for collecting a capillary specimen for a point-of-care glucose measurement?

  1. Use the first drop of blood without wiping
  2. Collect from a cool, cyanotic fingertip
  3. Wipe away the first drop and avoid excessive squeezing Correct answer.
  4. Squeeze the fingertip firmly to obtain a larger drop

Explanation

Wiping away the initial drop removes tissue fluid, and avoiding strong squeezing prevents dilution with interstitial fluid and hemolysis. Warm, well-perfused sites are preferred. Squeezing forcefully or sampling a cyanotic finger yields unreliable results.

Question 18Pre-analytical considerations: specimen collection, handling, and rejection criteria

A plasma sample for ammonia is transported on ice and analyzed after a delay. Which statement best reflects the pre-analytical requirement?

  1. Hemolysis has no effect on ammonia measurement
  2. The specimen should be warmed to body temperature before analysis
  3. Ammonia is stable at room temperature for hours, so ice is unnecessary
  4. The specimen should be placed on ice and separated and analyzed promptly to limit in vitro generation Correct answer.

Explanation

Ammonia can be generated in vitro by red cell and protein metabolism, so prompt separation and analysis, with cooling during transport, limit falsely elevated results. Warming would accelerate generation. Hemolysis also releases intracellular ammonia and interferes.

Question 19Pre-analytical considerations: specimen collection, handling, and rejection criteria

A laboratory receives a request for a therapeutic drug level drawn at an inappropriate time relative to dosing. What is the best pre-analytical response?

  1. Delay analysis until the next scheduled dose
  2. Reject the specimen because timing cannot be assessed
  3. Report the result with no comment because the assay is analytically valid
  4. Contact the ordering provider to clarify timing and document the collection time Correct answer.

Explanation

Therapeutic drug monitoring depends on knowing the collection time relative to dosing, so the laboratory should clarify timing and document it rather than silently reporting a potentially uninterpretable result. Rejection is not automatic if the time can be established. Holding the specimen until the next dose does not correct the timing problem.

Question 20Pre-analytical considerations: specimen collection, handling, and rejection criteria

A serum specimen for a lipid panel is collected from a patient who was not fasting, but the ordering information does not state fasting status. What is the most appropriate laboratory action?

  1. Hold the specimen until the patient confirms fasting
  2. Dilute the specimen to correct for postprandial turbidity
  3. Report the results and note that fasting status was not documented Correct answer.
  4. Reject the specimen because all lipid testing requires fasting

Explanation

Fasting requirements vary by analyte and clinical purpose, and the laboratory cannot assume non-fasting status is unacceptable. The appropriate action is to report results while documenting that fasting status was not recorded, allowing the clinician to interpret them. Automatic rejection or dilution is not justified.

Question 21Pre-analytical considerations: specimen collection, handling, and rejection criteria

A microbiology laboratory receives a swab for anaerobic culture that was transported in a standard aerobic transport medium and took two hours to arrive. What is the correct assessment?

  1. The specimen is suitable because anaerobic organisms survive in any medium
  2. The specimen is suitable if the swab remained moist
  3. The specimen is suitable if cultured immediately on arrival
  4. The specimen is likely compromised and should be rejected or recollected with anaerobic transport Correct answer.

Explanation

Anaerobic organisms are oxygen-sensitive and require transport systems that exclude oxygen; a standard aerobic medium plus delay compromises viability. Immediate culture cannot reverse exposure that already occurred during transport. Proper collection and transport media are essential for meaningful anaerobic culture.

Question 22Pre-analytical considerations: specimen collection, handling, and rejection criteria

Which pre-analytical variable most directly explains a discrepancy between two glucose results from the same patient drawn at the same time into a plain serum tube and a tube containing sodium fluoride?

  1. Differences in needle gauge between the two draws
  2. Random analytical imprecision of the glucose method
  3. The fluoride tube's larger draw volume
  4. Ongoing glycolysis in the serum tube before separation Correct answer.

Explanation

Fluoride inhibits glycolysis, whereas cells in a plain serum tube continue to consume glucose until separation, lowering the measured value. This produces a real pre-analytical difference between the two tubes. Needle gauge and draw volume do not explain a systematic glucose discrepancy of this kind.

Question 23Pre-analytical considerations: specimen collection, handling, and rejection criteria

A laboratory is designing a rejection policy for specimens with insufficient volume in pediatric EDTA tubes. Which approach best balances patient safety and analytical validity?

  1. Reject all underfilled EDTA tubes regardless of the test ordered
  2. Accept all underfilled tubes and report results with a disclaimer
  3. Define minimum volumes per test and use validated microvolume methods where available Correct answer.
  4. Require a repeat venipuncture for every underfilled tube without exception

Explanation

A well-designed policy specifies the minimum volume needed for each assay and uses validated small-volume methods when appropriate, avoiding unnecessary repeat collections in children. Blanket acceptance risks inaccurate results from excessive EDTA, while blanket rejection ignores validated alternatives. Disclaimers do not correct an analytically invalid sample.

Question 24Pre-analytical considerations: specimen collection, handling, and rejection criteria

A specimen for cold agglutinin testing arrives at the laboratory after transport at room temperature. What is the pre-analytical concern?

  1. Room temperature stabilizes red cells and improves accuracy
  2. Agglutination occurring in vitro may falsely alter the measured titer Correct answer.
  3. The specimen must be hemolyzed before testing
  4. Warming during transport inactivates the antibodies permanently

Explanation

Cold agglutinins bind red cells at lower temperatures, so a specimen cooled during transport can show in vitro agglutination that confounds interpretation. Samples are typically kept warm until serum is separated. Room-temperature transport does not stabilize the analyte for this test, and hemolysis is not a prerequisite.

Question 25Pre-analytical considerations: specimen collection, handling, and rejection criteria

A laboratory notices that potassium results from a satellite clinic are consistently higher than results from the main hospital for the same patients. Review of clinic practice shows tubes are transported by pneumatic tube system. What is the most likely explanation?

  1. The satellite laboratory calibrates potassium incorrectly
  2. Patients at the clinic have higher baseline potassium
  3. Pneumatic transport causes mechanical trauma and hemolysis, raising potassium Correct answer.
  4. The clinic uses a different reference range for potassium

Explanation

Pneumatic tube transport can subject specimens to acceleration and impact forces that hemolyze red cells, releasing potassium and producing falsely elevated results. A systematic difference tied to transport method points to a pre-analytical cause. Reference ranges, patient mix, and calibration would not produce this specific pattern.

Question 26Pre-analytical considerations: specimen collection, handling, and rejection criteria

A laboratory is writing its specimen collection manual and needs to state the correct order of draw for venipuncture tubes. Which sequence is correct?

  1. Serum tube with clot activator, blood culture bottles, EDTA tube, sodium citrate tube, heparin tube
  2. Blood culture bottles, sodium citrate tube, serum tube with clot activator, heparin tube, EDTA tube Correct answer.
  3. Heparin tube, EDTA tube, sodium citrate tube, serum tube with clot activator, blood culture bottles
  4. EDTA tube, sodium citrate tube, heparin tube, serum tube with clot activator, blood culture bottles

Explanation

The standard order of draw begins with sterile blood culture bottles, followed by sodium citrate coagulation tubes, then serum tubes with clot activator, heparin tubes, and finally EDTA tubes. The sequence minimizes carryover of additives such as EDTA and heparin into tubes whose tests are sensitive to those substances.

Question 27Pre-analytical considerations: specimen collection, handling, and rejection criteria

A phlebotomist must collect a specimen for a test that requires serum. Which tube type and handling step are appropriate for that specimen?

  1. A tube containing EDTA, mixed by inversion and centrifuged immediately after collection
  2. A tube with clot activator and serum separator gel, allowed to clot before centrifugation Correct answer.
  3. A tube containing sodium heparin, centrifuged within five minutes of collection
  4. A tube containing sodium citrate, filled to the marked volume and centrifuged at once

Explanation

Serum is obtained from blood that has been allowed to clot in a tube without anticoagulant, commonly one with clot activator and a separator gel. Complete clotting before centrifugation is necessary to yield cell-free serum and avoid fibrin strands in the sample.

Question 28Pre-analytical considerations: specimen collection, handling, and rejection criteria

A blood specimen is drawn from the arm in which an intravenous line is infusing fluid containing glucose and potassium. Which collection approach best protects the integrity of the chemistry results?

  1. Draw from the same line after briefly pausing the infusion for ten seconds
  2. Draw from the opposite arm when possible, or from below the line after flushing and discarding a suitable volume Correct answer.
  3. Collect the specimen from the line and note the infusion rate on the requisition
  4. Draw from the line's distal port without any flushing or discard step

Explanation

Specimens drawn from a site near an infusing line can be contaminated by the infusate, producing falsely elevated results for the substances being administered. The preferred approach is to collect from the other arm, or, when a line must be used, to flush the line and discard an adequate volume before collecting the specimen.

Question 29Pre-analytical considerations: specimen collection, handling, and rejection criteria

A capillary blood specimen is needed from the heel of a newborn for a metabolic screen. Which technique is correct?

  1. Puncture the central area of the heel pad after warming the skin
  2. Squeeze the heel firmly to obtain a large drop after puncture
  3. Puncture the posterior curvature of the heel, avoiding the central weight-bearing area Correct answer.
  4. Use the plantar surface near the arch to obtain the deepest puncture

Explanation

Heel punctures should be made on the medial or lateral portion of the plantar surface, avoiding the central weight-bearing area and the posterior curvature, which risk injury to underlying structures. The site should be warmed, punctured once, and the blood allowed to flow freely into the collection device.

Question 30Pre-analytical considerations: specimen collection, handling, and rejection criteria

A laboratory is reviewing a serum specimen for a test that is highly sensitive to hemolysis and to prolonged contact with cells. The specimen arrived in a plain tube without separator gel, was kept at room temperature, and was centrifuged six hours after collection. Which statement best describes the effect on the result?

  1. The result is unreliable because the delayed separation allowed analytes to shift between cells and serum Correct answer.
  2. The result is valid provided the tube was mixed by inversion before centrifugation
  3. The result is valid if the laboratory repeats the test on the same specimen after recentrifugation
  4. The delay and cell contact are acceptable because the tube contained no anticoagulant

Explanation

For analytes sensitive to cell contact, serum or plasma must be separated from cells promptly after clotting, because prolonged contact allows intracellular constituents to leak into the fluid phase and extracellular analytes to be consumed. Centrifuging hours later cannot reverse these shifts, so the specimen should be rejected and recollected when the analyte is critical.

Question 31Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A sputum specimen is submitted for bacterial culture. Which culture medium is selective for gram-negative rods by inhibiting gram-positive organisms while also differentiating lactose fermenters from non-fermenters?

  1. Columbia colistin-nalidixic acid agar
  2. Blood agar
  3. MacConkey agar Correct answer.
  4. Chocolate agar

Explanation

MacConkey agar contains bile salts and crystal violet that suppress gram-positive bacteria, and lactose plus neutral red that reveal lactose fermentation by pink colonies. Blood and chocolate agars are non-selective, and CNA agar inhibits gram-negatives rather than selecting for them.

Question 32Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A gram-negative bacillus recovered from a wound is tested with oxidase reagent. What color development indicates a positive oxidase reaction?

  1. Deep purple within 10 to 30 seconds Correct answer.
  2. Colorless zone around the colony
  3. Pink after adding Kovac reagent
  4. Yellow after 24 hours of incubation

Explanation

Oxidase reagent (tetramethyl-p-phenylenediamine) is oxidized by cytochrome c oxidase to a dark purple or blue-black color within seconds. Yellow after incubation and pink with Kovac reagent describe indole testing, not oxidase.

Question 33Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A gram-positive coccus grows in clusters and is catalase positive. Which organism is most consistent with these findings?

  1. Streptococcus pneumoniae
  2. Enterococcus faecalis
  3. Staphylococcus aureus Correct answer.
  4. Streptococcus pyogenes

Explanation

Staphylococci are gram-positive cocci in clusters that produce catalase, which converts hydrogen peroxide to water and oxygen. Streptococci and enterococci are catalase negative and typically form chains or pairs.

Question 34Bacteriology: isolation, identification, and antimicrobial susceptibility testing

What is the standard purpose of subculturing isolated colonies onto a non-selective medium before performing antimicrobial susceptibility testing?

  1. To induce resistance genes before exposure to antimicrobials
  2. To convert the organism into a standardized type strain
  3. To increase the mutation rate so resistance is detectable
  4. To obtain a pure, actively growing culture for reproducible inoculum preparation Correct answer.

Explanation

Susceptibility testing requires a pure culture in log-phase growth so that the standardized inoculum density is accurate and results reflect a single organism. Testing a mixed or stationary-phase culture produces unreliable and non-reproducible minimum inhibitory concentrations.

Question 35Bacteriology: isolation, identification, and antimicrobial susceptibility testing

Which result defines a bacterium as susceptible by broth microdilution?

  1. The minimum inhibitory concentration equals the resistance breakpoint
  2. The minimum inhibitory concentration is at or below the susceptible breakpoint Correct answer.
  3. The zone of inhibition is absent around the disk
  4. Growth is visible in every well at all tested concentrations

Explanation

Broth microdilution reports the lowest concentration that inhibits visible growth, and that MIC is interpreted against species-specific clinical breakpoints. An MIC at or below the susceptible breakpoint predicts likely clinical response.

Question 36Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A cerebrospinal fluid specimen from a patient with suspected meningitis is plated. Which medium best supports growth of fastidious organisms such as Haemophilus influenzae and Neisseria meningitidis?

  1. Potassium tellurite agar
  2. MacConkey agar incubated aerobically
  3. Chocolate agar incubated in increased carbon dioxide Correct answer.
  4. Selenite broth subcultured to blood agar

Explanation

Chocolate agar provides heat-lysed blood with released hemin and NAD, supporting fastidious respiratory and meningeal pathogens, and added carbon dioxide improves recovery. MacConkey agar inhibits gram-positives and does not enrich these organisms.

Question 37Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A stool isolate is a lactose-negative, oxidase-negative gram-negative rod that produces colorless colonies on MacConkey agar. Which genus is most consistent with these features?

  1. Escherichia
  2. Klebsiella
  3. Pseudomonas
  4. Salmonella Correct answer.

Explanation

Salmonella is a lactose-negative, oxidase-negative enterobacterium that forms pale, colorless colonies on MacConkey agar. Escherichia and Klebsiella ferment lactose, and Pseudomonas is oxidase positive.

Question 38Bacteriology: isolation, identification, and antimicrobial susceptibility testing

In disk diffusion testing, which factor most directly determines the size of the zone of inhibition for a given organism and antimicrobial?

  1. The number of biochemical tests performed on the isolate
  2. The color of the colony on the plate
  3. The species name assigned to the isolate
  4. The rate of antimicrobial diffusion through the agar relative to bacterial growth Correct answer.

Explanation

Zone size reflects the balance between antimicrobial diffusion through the agar and the growth rate of the organism, along with the initial inoculum and drug concentration in the disk. Colony color and biochemical profile do not change diffusion kinetics.

Question 39Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A gram-negative rod from a urine culture is oxidase positive, grows at 42 degrees Celsius, and produces a blue-green pigment. Which organism is most likely?

  1. Escherichia coli
  2. Salmonella enterica
  3. Proteus mirabilis
  4. Pseudomonas aeruginosa Correct answer.

Explanation

Pseudomonas aeruginosa is oxidase positive, grows at 42 degrees Celsius, and produces pyocyanin, a blue-green pigment. The other enteric organisms listed are oxidase negative and do not grow reliably at that temperature.

Question 40Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A beta-hemolytic, gram-positive coccus in chains is bacitracin sensitive and pyrrolidonyl arylamidase positive. Which organism does this identify?

  1. Enterococcus faecalis
  2. Streptococcus pyogenes Correct answer.
  3. Streptococcus agalactiae
  4. Streptococcus bovis

Explanation

Streptococcus pyogenes is bacitracin sensitive and PYR positive, which separates it from other beta-hemolytic streptococci. Streptococcus agalactiae is bacitracin resistant and PYR negative, and enterococci are PYR positive but not beta-hemolytic.

Question 41Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A coagulase-negative, novobiocin-resistant gram-positive coccus in clusters is recovered from a urine culture. Which species is most likely?

  1. Staphylococcus aureus
  2. Staphylococcus lugdunensis
  3. Staphylococcus saprophyticus Correct answer.
  4. Staphylococcus epidermidis

Explanation

Staphylococcus saprophyticus is a coagulase-negative staphylococcus distinguished by resistance to novobiocin, and it is a common cause of urinary tract infection. Staphylococcus epidermidis and S. lugdunensis are novobiocin susceptible.

Question 42Bacteriology: isolation, identification, and antimicrobial susceptibility testing

Which organism is intrinsically resistant to vancomycin because it possesses a modified peptidoglycan precursor?

  1. Enterococcus gallinarum Correct answer.
  2. Enterococcus faecalis
  3. Streptococcus pneumoniae
  4. Staphylococcus aureus

Explanation

Enterococcus gallinarum and E. casseliflavus carry the vanC gene, producing a modified peptidoglycan precursor that lowers vancomycin affinity and confers intrinsic low-level resistance. Enterococcus faecalis is usually vancomycin susceptible, and acquired resistance in S. aureus and pneumococci arises through other mechanisms.

Question 43Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A sputum isolate is a gram-negative coccobacillus that grows on chocolate agar but not on blood agar, and requires both hemin and NAD. Which organism is most consistent?

  1. Haemophilus parainfluenzae
  2. Haemophilus influenzae Correct answer.
  3. Neisseria meningitidis
  4. Moraxella catarrhalis

Explanation

Haemophilus influenzae requires both X factor (hemin) and V factor (NAD), so it grows on chocolate agar but fails on unsupplemented blood agar. Haemophilus parainfluenzae requires only V factor, and the other organisms do not have this dual requirement.

Question 44Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A stool isolate is oxidase negative and urease positive, and it swarms across the surface of a non-selective agar plate. Which organism is most likely?

  1. Shigella sonnei
  2. Proteus mirabilis Correct answer.
  3. Klebsiella pneumoniae
  4. Yersinia enterocolitica

Explanation

Proteus mirabilis is a lactose-negative, oxidase-negative enterobacterium that is strongly urease positive and shows characteristic swarming motility on agar. The other organisms listed are urease negative or do not swarm.

Question 45Bacteriology: isolation, identification, and antimicrobial susceptibility testing

When preparing a inoculum for disk diffusion testing, why is a 0.5 McFarland standard used?

  1. To neutralize antimicrobials carried over from the specimen
  2. To standardize the bacterial density so zone diameters are reproducible Correct answer.
  3. To provide nutrients that enhance antimicrobial diffusion
  4. To select for resistant subpopulations in the inoculum

Explanation

The 0.5 McFarland standard corresponds to approximately 1 to 2 times 10^8 colony-forming units per milliliter, and matching it ensures comparable inoculum density between runs. Without standardization, zone diameters vary and breakpoint interpretation becomes unreliable.

Question 46Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A gram-positive rod from a blood culture is catalase positive and shows tumbling motility at room temperature. Which organism is most likely?

  1. Corynebacterium diphtheriae
  2. Clostridium perfringens
  3. Bacillus cereus
  4. Listeria monocytogenes Correct answer.

Explanation

Listeria monocytogenes is a catalase-positive gram-positive rod that exhibits characteristic tumbling motility at room temperature. Corynebacterium is catalase positive but shows club-shaped palisades without tumbling motility, and Clostridium is anaerobic.

Question 47Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A urine isolate is a gram-negative rod that grows on MacConkey agar as a mucoid lactose fermenter and is indole negative, citrate positive, and urease positive. Which organism is most likely?

  1. Klebsiella pneumoniae Correct answer.
  2. Escherichia coli
  3. Enterobacter cloacae
  4. Proteus vulgaris

Explanation

Klebsiella pneumoniae is a non-motile, encapsulated lactose fermenter that is indole negative, citrate positive, and urease positive, producing mucoid colonies. Escherichia coli is indole positive, and Proteus vulgaris is lactose negative.

Question 48Bacteriology: isolation, identification, and antimicrobial susceptibility testing

Which method is appropriate for detecting methicillin resistance in Staphylococcus aureus when a cefoxitin disk is used?

  1. Incubate at 42 degrees Celsius to accelerate growth
  2. Read the zone after 4 hours of incubation
  3. Incubate at 33 to 35 degrees Celsius for a full 24 hours and read the zone Correct answer.
  4. Add sodium chloride to the medium and read after 6 hours

Explanation

Cefoxitin disk testing for mecA-mediated resistance requires incubation at 33 to 35 degrees Celsius for a full 24 hours because expression of penicillin-binding protein 2a is temperature and time dependent. Shortened incubation or elevated temperature can cause false-susceptible results.

Question 49Bacteriology: isolation, identification, and antimicrobial susceptibility testing

A blood culture isolate is a gram-positive coccus in pairs that is optochin sensitive and bile soluble. Which organism is identified?

  1. Enterococcus faecium
  2. Streptococcus pneumoniae Correct answer.
  3. Streptococcus agalactiae
  4. Streptococcus mitis

Explanation

Streptococcus pneumoniae is optochin sensitive and lyses in bile salts, which distinguishes it from viridans streptococci such as S. mitis. Enterococcus is optochin resistant, and S. agalactiae is beta-hemolytic and bacitracin resistant.

Question 50Bacteriology: isolation, identification, and antimicrobial susceptibility testing

An isolate is reported as resistant to a carbapenem by automated susceptibility testing. Which confirmatory approach best distinguishes carbapenemase production from other resistance mechanisms?

  1. Perform a carbapenemase inactivation assay such as the modified carbapenem inactivation method Correct answer.
  2. Repeat the same automated panel to confirm the result
  3. Perform a coagulase test on the isolate
  4. Test for inducible clindamycin resistance with a D-zone test

Explanation

Phenotypic carbapenemase detection, such as the modified carbapenem inactivation method, directly demonstrates enzymatic inactivation of a carbapenem disk and separates carbapenemase producers from organisms resistant through other means. Repeating the same panel confirms the MIC but not the mechanism.