Q- A patient with chronic kidney disease develops secondary hyperparathyroidism. Which of the following biochemical changes directly stimulates parathyroid hormone (PTH) secretion in this condition?
A- Decreased 1,25-dihydroxyvitamin D levels
B- Decreased serum phosphate
C- Increased calcium-sensing receptor activation
D- Increased intestinal calcium absorption
Q- A mutation prevents phosphorylation of the regulatory light chain of myosin in smooth muscle. Which immediate cellular effect is most likely?
A- Decreased actin-myosin cross-bridge formation
B- Increased calcium release from sarcoplasmic reticulum
C- Increased troponin C binding
D- Normal contraction with increased ATP consumption
Q- A child presents with recurrent viral infections. Flow cytometry shows absent MHC class I expression on all nucleated cells. Which immune defect best explains this finding?
A- Defective beta-2 microglobulin synthesis
B- Impaired CD4+ T-cell differentiation
C- Impaired TAP transporter function
D- Loss of thymic epithelial cells
Q- A drug irreversibly inhibits an enzyme by forming a covalent bond at the active site. Which pharmacologic change will restore maximal enzyme activity?
A- Increasing substrate concentration
B- Increasing enzyme concentration
C- Decreasing competitive inhibitors
D- Removing allosteric modulators
Q- A patient has a mutation that prevents NADH oxidation in the mitochondrial electron transport chain. Which metabolic consequence is most likely?
A- Decreased ATP production from oxidative phosphorylation
B- Increased flux through the citric acid cycle
C- Increased mitochondrial membrane potential
D- Reduced lactate formation