MLT GlobeChemistry: Carbohydrates & diabetes – page 6
121 Chemistry MCQs on Carbohydrates & diabetes with answers and explanations.
A diabetic patient with nephrotic syndrome (albumin 2.0 g/dL, 20 g/L) has a low fructosamine that does not match high home glucose readings. Why?
Fructosamine measures glycated serum proteins, mostly albumin. Low albumin or fast protein turnover gives falsely low results. Fructosamine reflects the previous 2–3 weeks, not 3 months.
A patient with renal failure taking metformin has lactate 9 mmol/L (81 mg/dL), normal blood pressure and normal oxygen saturation. This is best classified as:
Type B lactic acidosis occurs without tissue hypoxia, for example from drugs such as metformin. Type A results from poor tissue oxygen delivery, such as shock or severe hypoxemia.
In a polarographic glucose oxidase method, the glucose concentration is proportional to:
Glucose oxidase uses oxygen to oxidize glucose; an oxygen electrode measures how fast O2 is consumed. The quinoneimine dye is the end point of the colorimetric (Trinder) version.
An adult with chronic diarrhea and steatorrhea has low blood and urine D-xylose after an oral dose. Renal function is normal. This most suggests:
D-xylose is absorbed without needing pancreatic enzymes or bile, so poor absorption points to a mucosal problem (e.g., celiac disease). In pancreatic insufficiency the xylose test is normal.
A diabetic patient's treatment was changed 3 weeks ago. Which test best shows the effect of this change on average glucose?
Fructosamine (glycated serum proteins, mainly albumin) reflects average glucose over the previous 2–3 weeks. HbA1c reflects about 2–3 months, so it responds more slowly to a recent change.
A patient with known diabetes on treatment has an HbA1c of 9.0% (75 mmol/mol). This result indicates:
HbA1c reflects average glucose over the red cell lifespan (about 2–3 months). A value of 9% is well above the usual target of below 7%, indicating poor control over that period.
An adult has a fasting plasma glucose of 140 mg/dL (7.8 mmol/L) on two separate days. By ADA criteria this indicates:
A fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or higher, confirmed on repeat testing, meets ADA criteria for diabetes. Impaired fasting glucose is 100–125 mg/dL (5.6–6.9 mmol/L).
Glycogen branching enzyme removes a terminal block of about how many glucose residues and reattaches it by an α-1,6 bond?
Once a chain reaches at least 11 residues, the branching enzyme moves a segment of 6–7 glucose units to form a new branch at least four residues from the previous one.
Glucose oxidase reacts only with beta-D-glucose. Which enzyme is added to some reagents to speed up conversion of alpha-D-glucose to the beta form?
Mutarotase speeds the conversion of alpha- to beta-D-glucose so all glucose is measured quickly. Hexokinase is the enzyme of a different glucose method.
Why can glucose still fall during the first hour after collection in a sodium fluoride tube?
Fluoride inhibits enolase, a late step of glycolysis, and takes 1–4 hours to be fully effective. Placing the tube in ice slurry or using a citrate-buffered tube gives immediate protection.
A pregnant woman at 26 weeks has a 75 g OGTT: fasting 95 mg/dL, 1 h 170 mg/dL, 2 h 140 mg/dL. Using the one-step (IADPSG) criteria, the result is:
One-step thresholds are fasting ≥92, 1 h ≥180 and 2 h ≥153 mg/dL; any single value met is diagnostic. Needing two abnormal values applies to the two-step (Carpenter–Coustan) 100 g test.
Using eAG (mg/dL) = 28.7 × HbA1c − 46.7, what is the estimated average glucose for an HbA1c of 7.0%?
28.7 × 7.0 = 200.9; 200.9 − 46.7 = 154 mg/dL (about 8.6 mmol/L). 183 mg/dL corresponds to an HbA1c of about 8%.
A patient in DKA improves with treatment, yet the nitroprusside ketone test becomes more strongly positive. The best explanation is:
In DKA most ketone is beta-hydroxybutyrate, which nitroprusside does not detect. With treatment it is oxidised to acetoacetate, so the test rises. Measuring beta-hydroxybutyrate avoids this problem.
A patient has fasting hypoglycemia with high insulin, high C-peptide and high proinsulin. Before diagnosing insulinoma, which test is essential?
Sulfonylureas stimulate beta cells and give the same biochemical pattern as insulinoma, so a drug screen is needed. Anti-insulin antibodies alone do not exclude drug use.
An infant has fasting hypoglycemia, lactic acidosis, hyperuricemia, hypertriglyceridemia and a large liver. No rise in glucose follows glucagon. Which enzyme is most likely deficient?
Glucose-6-phosphatase deficiency (von Gierke, GSD type I) blocks glucose release from liver glycogen and gluconeogenesis, causing this pattern. Muscle phosphorylase deficiency (McArdle) affects muscle, not fasting glucose.
A pregnant woman fails the 50 g screen. Her 100 g 3-hour OGTT: fasting 90, 1 h 185, 2 h 160, 3 h 130 mg/dL. Using Carpenter–Coustan limits (95/180/155/140 mg/dL), the result is:
In the two-step approach, gestational diabetes is diagnosed when at least two of the four values meet or exceed the limits. Here the 1-hour and 2-hour values are abnormal.
A 20-year-old has muscle cramps and dark urine after exercise. In a forearm exercise test, lactate fails to rise while ammonia rises normally. The most likely disorder is:
Without muscle phosphorylase, glycogen cannot be broken down for glycolysis, so lactate does not rise. In myoadenylate deaminase deficiency the pattern is reversed: lactate rises but ammonia does not.
Measured sodium is 130 mmol/L with glucose 600 mg/dL (33.3 mmol/L). Correcting by 1.6 mmol/L sodium for each 100 mg/dL glucose above 100 mg/dL, the corrected sodium is about:
Glucose is 500 mg/dL above 100, so 5 × 1.6 = 8; 130 + 8 = 138 mmol/L. Hyperglycemia pulls water out of cells and dilutes sodium; this is a real (translocational) change, not an analytical artifact.
A patient heterozygous for a hemoglobin variant has an unreliable HbA1c by ion-exchange HPLC. Which method is least affected by most hemoglobin variants?
Boronate affinity binds the glucose adduct itself, not the charge of the hemoglobin, so most variants do not interfere. Charge-based methods can mis-separate variants. No method is valid if red cell survival is shortened.
A 65-year-old with a large mesenchymal tumor has fasting hypoglycemia with low insulin, low C-peptide, low β-hydroxybutyrate and a clear glucose rise after glucagon. The most likely cause is:
IGF-2 acts on insulin receptors, suppressing insulin, C-peptide and ketogenesis while preserving liver glycogen. Insulinoma raises insulin and C-peptide; injected insulin raises insulin.