Which of the following is not true regarding sickle-cell disease?
It is a cause of anemia.
It is cause of malaria.
It is due to a hereditary hemoglobin defect
It is advantageous in that it can protect carriers against malaria
It is caused by excessive allele that modifies the structure of hemoglobin
The Correct Answer is B
A. It is a cause of anemia: Sickle cells are fragile and hemolyze prematurely, producing a chronic hemolytic anemia.
B. It is cause of malaria: Malaria is caused by Plasmodium parasites; sickle-cell disease does not cause malaria (in fact, sickle-cell trait provides some protection against severe malaria).
C. It is due to a hereditary hemoglobin defect: Sickle-cell disease is inherited and results from a mutation in the β-globin (HBB) gene producing hemoglobin S.
D. It is advantageous in that it can protect carriers against malaria: Heterozygous carriers (sickle-cell trait) have relative resistance to severe forms of Plasmodium falciparum malaria.
E. It is caused by excessive allele that modifies the structure of hemoglobin: The disease is caused by a point mutation (a single nucleotide change) in the β-globin allele that alters hemoglobin structure (Glu→Val at position 6). Saying “excessive allele” is misleading -the cause is a specific mutant allele, not an “excessive” one.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
A. A chemical messenger that is triggered by the binding of a ligand on the cell surface and causes a change in DNA transcription: This describes how some hormones act inside a cell, but it is not the definition of a hormone.
B. A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away: This is the textbook definition of a hormone: an endocrine signal transported in the blood to distant target organs.
C. Chemical messengers that travel across a synapse to stimulate another cell: This describes neurotransmitters, not hormones.
D. Chemical messengers that diffuse from their point of origin locally to affect other cells physiology: This describes paracrine or autocrine signals, not hormones.
Correct Answer is {"dropdown-group-1":"D"}
Explanation
A. thymus: The thymus secretes thymosins that influence T-cell maturation (immune function), not hormones that raise blood calcium - so it does not respond to hypocalcemia.
B. thyroid gland: The thyroid’s parafollicular (C) cells secrete calcitonin, which lowers blood calcium in response to hypercalcemia, not hypocalcemia.
C. pituitary gland: The pituitary releases many trophic hormones, but it does not directly secrete the primary hormone (PTH) that corrects hypocalcemia.
D. parathyroid glands: The parathyroid glands secrete parathyroid hormone (PTH) in response to low blood Ca²⁺; PTH raises serum calcium by acting on bone, kidney, and vitamin D activation.
E. pineal gland: The pineal gland secretes melatonin to regulate circadian rhythms, not calcium homeostasis.
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