MLT GlobeDOH exam preparation (Abu Dhabi) – page 19
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
Which enzyme catalyses a reversible step used in both glycolysis and gluconeogenesis?
Phosphoglycerate kinase is near-equilibrium and works in both directions. Pyruvate kinase and hexokinase are irreversible glycolytic steps; fructose-1,6-bisphosphatase is used only in gluconeogenesis.
Glucose-6-phosphate is converted to fructose-1,6-bisphosphate in two steps. Which pair of enzymes is involved?
Phosphoglucose isomerase forms fructose-6-phosphate, which PFK-1 phosphorylates. Phosphoglucomutase interconverts glucose-6- and glucose-1-phosphate; aldolase acts after fructose-1,6-bisphosphate is formed.
The sugar most often attached to ceramide in cerebrosides of brain myelin is:
Galactocerebroside is the main cerebroside of neural myelin. Glucocerebroside dominates outside the nervous system, and sialic acid characterises gangliosides.
Which phospholipid is concentrated mainly in the outer (extracellular) leaflet of the plasma membrane?
Choline phospholipids (phosphatidylcholine, sphingomyelin) face outward. Phosphatidylserine is kept on the inner leaflet; its exposure outside signals apoptosis and supports coagulation.
Which biological membrane has the highest ratio of protein to lipid?
The inner mitochondrial membrane is packed with electron-transport complexes and ATP synthase (about 75% protein). Myelin is the opposite extreme, mostly lipid for insulation.
Arterial blood has a pH of 7.40. Its hydrogen ion concentration is approximately:
[H+] = 10^-7.4 mol/L ≈ 4.0 × 10^-8 mol/L, i.e. 40 nmol/L. Each 0.3 pH unit change roughly doubles or halves this value.
Glucose is taken up by intestinal cells against its concentration gradient, coupled to sodium entry via SGLT1. This process is:
SGLT1 uses the Na+ gradient created by Na+/K+-ATPase to move glucose uphill. Diffusion, simple or facilitated, only moves solutes down their gradient.
Hay's sulfur test detects bile salts in urine because bile salts:
Bile salts are detergents that reduce surface tension, the same property that emulsifies fat in the intestine. Sulfur powder therefore sinks instead of floating; no chemical reaction is involved.
Which monosaccharide shares the SGLT1 transporter with glucose and is absorbed by active transport in the small intestine?
SGLT1 co-transports glucose and galactose with sodium. Fructose is absorbed by facilitated diffusion through GLUT5.
Which glycosaminoglycan is most characteristic of the corneal stroma, helping keep the cornea transparent?
Keratan sulfate proteoglycans regulate collagen fibril spacing in the cornea. Heparin is in mast cells, heparan sulfate on cell surfaces and basement membranes, and hyaluronate in vitreous and synovial fluid.
The immunodominant sugar that defines the blood group A antigen is:
A transferase adds N-acetylgalactosamine to H substance; B transferase adds galactose. Fucose forms the H antigen itself, present on all common groups.
Which antibacterial drug blocks bacterial folate synthesis by competing with p-aminobenzoic acid (PABA)?
Sulfonamides are PABA analogues that inhibit dihydropteroate synthase. Trimethoprim acts later on bacterial dihydrofolate reductase, and methotrexate targets human DHFR in cancer therapy.
Using current P/O ratios, oxidation of one mitochondrial NADH through the electron transport chain yields about:
Current estimates give about 2.5 ATP per NADH and 1.5 per FADH2, because proton pumping and ATP synthase stoichiometry are non-integral. The older whole-number value was 3.
An enzyme has a Km of 2 mmol/L. At a substrate concentration of 6 mmol/L, the velocity is what fraction of Vmax?
v/Vmax = [S]/(Km + [S]) = 6/8 = 0.75. Velocity equals 50% of Vmax only when [S] equals Km.
Which metabolite inhibits carnitine palmitoyltransferase-I, preventing newly made fatty acids from being oxidised?
Malonyl-CoA, the first committed intermediate of fatty acid synthesis, blocks CPT-I so synthesis and oxidation do not run together. Acetyl-CoA is a substrate, not the CPT-I inhibitor.
Dopamine β-hydroxylase, which converts dopamine to norepinephrine, requires which vitamin as a reductant?
This copper enzyme uses ascorbate to keep copper reduced during hydroxylation. Vitamin C also serves prolyl and lysyl hydroxylases in collagen synthesis.
A patient has raised serum calcium, low phosphate and raised parathyroid hormone. The most likely diagnosis is:
Excess PTH raises calcium and increases phosphate excretion. Osteomalacia and rickets cause low or normal calcium with secondary PTH rise; hypoparathyroidism causes hypocalcaemia.
Methylmalonyl-CoA mutase, whose failure raises urine methylmalonic acid, requires which coenzyme form of vitamin B12?
Deoxyadenosylcobalamin is the mutase cofactor; methylcobalamin serves methionine synthase. Cyano- and hydroxocobalamin are pharmaceutical forms that must be converted.
Which vitamin-derived coenzyme is NOT used by the α-ketoglutarate dehydrogenase complex?
The complex uses thiamine pyrophosphate, lipoate, CoA, FAD and NAD+, like pyruvate dehydrogenase. Tetrahydrofolate carries one-carbon units in other pathways.
Vasopressin acting on V1 receptors of vascular smooth muscle raises intracellular calcium through:
V1 receptors are Gq-coupled and activate phospholipase C; IP3 releases stored calcium. The renal V2 receptor, by contrast, uses Gs and cAMP to insert aquaporin-2.