MLT GlobeChemistry: Proteins & electrophoresis – page 6
125 Chemistry MCQs on Proteins & electrophoresis with answers and explanations.
A newborn screening test shows high blood phenylalanine. The classic form of this disorder is caused by deficiency of:
Classic phenylketonuria is phenylalanine hydroxylase deficiency, which blocks conversion of phenylalanine to tyrosine. Tyrosinase deficiency causes albinism.
A newborn with poor feeding, seizures and sweet-smelling urine has high plasma leucine, isoleucine, valine and alloisoleucine. Which enzyme is deficient?
Maple syrup urine disease is caused by failure to decarboxylate the keto acids of the branched-chain amino acids. Alloisoleucine is diagnostic. The keto acids give the urine its maple-syrup smell.
An adult's urine turns black on standing; he also has dark ear cartilage and early arthritis. Which substance accumulates?
Alkaptonuria is a homogentisate 1,2-dioxygenase deficiency. Homogentisic acid oxidizes to a black pigment in urine and deposits in connective tissue (ochronosis).
Newborn screening for many amino acid and fatty acid oxidation disorders from a single dried blood spot is performed by:
Tandem mass spectrometry measures many amino acids and acylcarnitines at once from one blood spot. The Guthrie test detected only phenylalanine and is now largely historical.
Which protein marker is used to detect sustained heavy alcohol intake over the previous 2–3 weeks?
Heavy drinking (roughly more than 50–80 g ethanol daily) reduces transferrin glycosylation, increasing carbohydrate-deficient transferrin. It normalizes after a few weeks of abstinence.
Clear fluid drips from the nose after a head injury. Which protein confirms that the fluid is CSF?
Beta-2 transferrin (asialotransferrin) is formed in the CNS by neuraminidase and is found in CSF and perilymph but not in nasal secretions or serum. It is detected by electrophoresis with immunofixation.
Compared with ESR, C-reactive protein is better for monitoring an acute infection because CRP:
CRP is made by the liver under IL-6 stimulation, rises within 6–8 hours and has a half-life of about 19 hours, so it falls quickly with recovery. ESR changes slowly and is affected by anemia and red cell shape.
A patient has total protein 9.5 g/dL (95 g/L) and albumin 3.0 g/dL (30 g/L). Which protein group is most likely increased?
Globulin = total protein − albumin = 6.5 g/dL, well above the usual 2.0–3.5 g/dL. A high total protein with normal-to-low albumin points to raised globulins, for example immunoglobulins.
A malnourished child has generalised edema. Which serum finding best explains the edema?
Albumin provides most of the plasma oncotic pressure. When albumin is low, fluid leaks from vessels into tissues, producing edema, as in kwashiorkor.
In serum protein electrophoresis at pH 8.6, what charge do most serum proteins carry and toward which electrode do they move?
At pH 8.6, above the isoelectric point of most serum proteins, they lose protons and become negatively charged, so they migrate toward the positive anode.
In bacteria and plants, methionine, threonine and lysine are all synthesised from which amino acid?
The aspartate family pathway leads to lysine, threonine and methionine (and isoleucine). Glutamate gives proline and arginine; serine gives glycine and cysteine.
Ubiquitin, the small protein that tags others for proteasomal breakdown, consists of how many amino acids?
Ubiquitin is a highly conserved 76-residue protein; its C-terminal glycine attaches to lysine residues of target proteins.
A serum sample with marked in-vitro hemolysis is used for protein electrophoresis. Which region is most likely to show a false increase?
Free hemoglobin and hemoglobin–haptoglobin complexes migrate in the alpha-2 to beta area and can look like an abnormal band there. Hemolysis does not add to the gamma fraction.
A 65-year-old with bone pain and renal failure has low gamma globulin and no visible spike on serum electrophoresis. Which tests best detect a light-chain-only myeloma?
Free light chains are small, cleared by the kidney and often invisible on serum electrophoresis, which may show only reduced gamma. Serum free light chain assay and urine immunofixation are the sensitive tests.
In a patient with normal renal function, the usual reference interval for the serum free kappa/lambda light chain ratio is about:
The commonly used reference range for the serum free kappa/lambda ratio is 0.26–1.65. A ratio outside this range suggests a clonal plasma-cell process; renal impairment widens the expected range.
Immunofixation shows a sharp band in the lambda lane only; the IgG, IgA and IgM lanes show no matching band. The best next step is:
A light-chain band with no matching G, A or M heavy chain is either free lambda or an uncommon IgD or IgE monoclonal protein. Testing with anti-IgD and anti-IgE separates these.
CSF IgG 9 mg/dL, serum IgG 1000 mg/dL, CSF albumin 20 mg/dL, serum albumin 4000 mg/dL. What is the IgG index and its meaning (reference ≤0.7)?
IgG index = (CSF IgG/serum IgG) ÷ (CSF albumin/serum albumin) = 0.009 ÷ 0.005 = 1.8. A value above about 0.7 suggests IgG production inside the CNS.
Capillary zone electrophoresis shows a small new peak in the beta region. Immunofixation is negative. The patient had a contrast CT scan the previous day. Most likely cause?
Capillary electrophoresis detects proteins by UV absorbance near 200–214 nm; radiocontrast agents absorb at these wavelengths and create artifactual peaks. A negative immunofixation argues against a monoclonal protein.
On agarose electrophoresis at pH 8.6, gamma globulins may move slightly toward the cathode, behind the application point. This is caused by:
The fixed negative charges of the support attract positive ions that flow toward the cathode, carrying water with them. This flow pulls weakly charged gamma globulins backward.
A serum stored for several days shows a smaller beta-2 band than when it was fresh. Which protein is most likely responsible?
C3 is unstable and breaks down during storage into fragments that move differently, so the beta-2 band shrinks. Fresh serum should be used for electrophoresis.