A father and mother are carriers of phenylketonuria (PKU). Their 2-year-old daughter has PKU.
The couple tells the nurse that they are planning to have a second baby.
Because their daughter has PKU, they are sure that their next baby won't be affected.
What is the best response by the nurse?
The ultrasound indicates a boy, and boys are not affected by PKU.
You are both carriers, so each baby has a 25% chance of being affected.
I think you'd better check with your doctor first.
Good planning: you need to take advantage of the odds in your favor.
The Correct Answer is B
Choice A rationale
Phenylketonuria (PKU) is an autosomal recessive genetic disorder, meaning it affects individuals regardless of their sex. The inheritance pattern of autosomal recessive traits dictates that both males and females have an equal probability of inheriting the two recessive alleles required for the expression of the disease. Therefore, gender has no bearing on the likelihood of a child being affected by PKU.
Choice B rationale
Phenylketonuria (PKU) is an autosomal recessive disorder. When both parents are carriers, each child has a 25% chance of inheriting two copies of the recessive gene (one from each parent) and therefore being affected by PKU. This probability applies independently to each pregnancy, regardless of the genetic status of previous children.
Choice C rationale
While consulting a doctor is generally good advice, this response does not provide the specific genetic information the parents need regarding the probability of their next child being affected by PKU. It avoids addressing the core question about the inheritance pattern and recurrence risk for an autosomal recessive disorder.
Choice D rationale
This response is misleading and scientifically inaccurate. The odds of having an affected child do not change with subsequent pregnancies for autosomal recessive disorders. Each pregnancy represents an independent event with the same 25% chance of the child inheriting PKU if both parents are carriers.
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Correct Answer is D
Explanation
Choice A rationale
This statement accurately describes the physiological process of nutrient and gas exchange in the placenta. Diffusion is the primary mechanism, driven by concentration gradients, where oxygen and nutrients move from the maternal blood into the fetal circulation, and waste products move from fetal to maternal blood. The placental barrier, though thin, ensures that maternal and fetal bloodstreams remain separate, preventing direct mixing. This separation is crucial for immunological compatibility.
Choice B rationale
This response demonstrates a correct understanding of placental anatomy and function. The chorionic villi are finger-like projections of the placenta that extend into the maternal decidua. They are rich in fetal capillaries and are the primary site of exchange between maternal and fetal blood. The close proximity allows for efficient transfer of essential substances like oxygen and nutrients, and removal of waste products.
Choice C rationale
This statement is physiologically accurate. The maternal and fetal circulatory systems are distinct; there is no direct mixing of blood. The placenta acts as an interface for exchange. The mother provides oxygen and nutrients, while the fetus returns metabolic waste products, including carbon dioxide and urea, to the maternal circulation for excretion. This maintains separate, yet interdependent, physiological systems.
Choice D rationale
This statement is incorrect and indicates a need for further teaching. The maternal and fetal bloodstreams do not directly mix. The placenta serves as a critical barrier and exchange organ, facilitating the transfer of substances via diffusion, active transport, and facilitated diffusion, but maintaining the separation of the two circulatory systems. Direct blood mixing would lead to severe immunological complications.
Correct Answer is C
Explanation
Choice A rationale
Human placental lactogen (hPL) is a hormone produced by the placenta during pregnancy. Its primary functions include modulating maternal metabolism to ensure nutrient supply to the fetus and promoting mammary gland development. It is not involved in stimulating ovarian follicle development for the purpose of inducing ovulation in fertility treatments.
Choice B rationale
Estrogen cream, typically estradiol, is a form of hormone replacement therapy used to alleviate symptoms of menopause, such as vaginal atrophy, by topical application. While estrogen plays a crucial role in follicular development, exogenous estrogen cream is not the primary medication used to stimulate ovarian follicle maturation in fertility protocols.
Choice C rationale
Clomiphene, a selective estrogen receptor modulator, works by blocking estrogen receptors in the hypothalamus and pituitary gland. This blockade prevents negative feedback, leading to an increase in gonadotropin-releasing hormone (GnRH) and subsequent surges in follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate ovarian follicular growth and ovulation.
Choice D rationale
Progestin, or progesterone, is a hormone primarily involved in preparing the uterine lining for implantation and maintaining pregnancy. In fertility treatments, it is often administered after ovulation or embryo transfer to support the luteal phase and enhance endometrial receptivity, not to stimulate the initial development of ovarian follicles.
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