USMLEGladiator
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15. MRL-lpr/lpr mutant mice are useful biomedical research models because of their increased susceptibility to autoimmune disease. These mice are characterized by a defect in the fas gene and a marked increase in T-lymphocyte accumulation as compared with wild-type animals. Based on these characteristics, which of the following is the most likely cause of the increased susceptibility of these mutants to autoimmune disease?
A. Decreased cell-cycle transit time for activated T lymphocytes
B. Decreased dependence on antigen-presenting cells for T-lymphocyte activation
C. Decreased responsiveness to clonal deletion signals in the thymic cortex
D. Increased responsiveness of mature T lymphocytes to antigenic stimulation
E. Increased thymocyte precursor proliferation in the bone marrow
F. Increased thymocyte proliferation rate in the thymic cortex
I think C
17. The P50 is defined as the partial pressure of oxygen (in mm Hg) required to saturate half of the hemoglobin O2 binding sites in a sample of whole blood. In blood from a healthy adult, the P50 is approximately 26 mm Hg when equilibrated with a mixture of 16% O2/5% CO2/79% N2. Which of the following manipulations will cause the P50 of the sample to increase?
A. Cooling the blood from 37°C to 20°C
B. Depleting its erythrocytes of 2,3-bisphosphoglycerate
C. Equilibrating it with 16% O2/12% CO2/72% N2
D. Increasing its pH by 0.3 units using NaOH
E. Mixing it 1:1 with cord blood from a newborn
I thnk A
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| mazinger
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I agree with you in the first question, Fas ligand is a death signal (apoptosis) and is widely used for clonal selection.. This is an intrauterine process for developing tolerance to our own antigens.. 2nd question I think theyre asking which of the following options produces a right shift of the curve or increases the p50; in a way these 2 mean the same.. I would go for C.. Option A would decrease the p50 increasing the affinity of hemoglobin for oxygen..
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original mazinger z
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| robin082006
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1 C
2 C for sure
right shift: increased temperature, increased PCO2, increased 23DPG, increased H+(decreased Ph)
left shift: changing the above factors in opposite way.
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| DrVirgo
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Can someone please explain #15 again? Its still confusing. C sounds right, but the other choices are distracting... how can we eliminate them?
Thanks. 
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| ord
Forum Junior
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1. C agree
2. C but still contradicting...
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| Cedrick
Forum Fanatic
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15.-C mutants mice are vulnerable to autoinmmune disease,because they lack {defect}
tolerance of their own antigens
17.-C for sure
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