Compound X is a pharmaceutical drug used to artificially stimulate increased bond density by artificially binding to CTRs (Calcitonin Receptors). Which of the following statements are true of Compound X?

I. Compound X binds to a GPCR in order to elicit its effects.

II. Compound X requires being bound to albumin in order to be transported through the blood.

III. Compound X directly binds to operator sites of DNA within bone cells in order to stimulate increased gene expression of calcitonin.

IV. Compound X inhibits PTH activity in order to increase blood calcium levels.

A. I only.

B. II and III.

C. I and III.

D. II, III, and IV.

Based on the passage data, which of the following is a likely effect of calcitonin overexpression in Group B rats?
A. Increased intestinal calcium absorption
B. Decreased serum calcium levels
C.Increased osteoclast activity
D. Decreased bone mineral density

Calcitriol exerts its effects of increasing serum calcium concentrations by primarily acting on which of the following tissues?
A. Parathyroid glands
B. Bone marrow
C. Intestinal epithelium
D. Hepatic sinusoids

A researcher hypothesizes that PTH indirectly increases intestinal calcium absorption. Which of the following experimental outcomes would most support this hypothesis?
A. PTH increases the number of osteoclasts in vitro
B. PTH administration raises blood calcium even when intestinal absorption is blocked
C. PTH raises serum calcium in rats only when vitamin D is active
D. PTH suppresses calcitonin expression in posterior pituitary tissue

Which of the following changes would be most expected in a patient with hypoparathyroidism?
A. Elevated serum calcium and increased osteoclast activity
B. Decreased serum calcium and increased calcitonin secretion
C. Elevated calcitriol levels and increased intestinal calcium absorption
D. Decreased serum calcium and reduced bone resorption

What would happen to serum concentrations of PTH and calcitonin, respectively, in the event that a patient is diagnosed with a cancer that causes over-active osteoblasts?

A. Increase, Decrease

B. Increase, Increase

C. Decrease, Increase

D. Decrease, Decrease

Answer: A

Content to know: Peptide hormones (which have names that are acronyms or end in “-in”, such as ACTH or calcitoninc), are polar/hydrophilic, and therefore can freely travel through the blood without any help from a carrier protein, bind to GPCRs on cell surfaces to elicit their effects, and have short but quick effects. Steroid hormones (which have names that end in -isol, -ogen, -erone) are nonpolar/hydrophobic, and therefore need carrier proteins to travel through the blood but can travel through membranes and have receptors intracellularly, including on DNA in some cases. Steroid hormones also have much longer lasting but slower affects.

Explanation:

My tip for roman numeral based questions: look for a roman numeral that shows up in two of the answer choices and start there. For example, option I shows up in two of the four answer choices, and if it’s correct, we can eliminate anything that doesn’t have option I in it. This will make going through the answer choices a lot easier, since as you can see in this question, if option I is correct then we only have to look at option III after eliminating B and D.

To actually go through this question, we will start with option I.

• Option I- Correct. Since this compound can bind to CTRs like calcitonin, we know it must have similar properties to peptide hormones. Therefore option I must be true.

From here, we can eliminate B and D, and now we only have to determine if option III is correct or not.

• Option III- Incorrect. A is the answer. As we just discussed, Compound X will act very similarly to a peptide hormone and will not bind to DNA (which a steroid hormone could do). This question seems like it could be a good answer since stimulating calcitonin gene expression would increase bone density like the question states, but that doesn’t matter since the first part of this option is scientifically incorrect, as peptide hormones do not bind directly to DNA. Never pick an answer choice that is scientifically incorrect!

Therefore the answer is A. To explain II and IV:

• Option II- Incorrect. If compound X is behaving as a peptide hormone analog of calcitonin, it’s polar and doesn’t need a carrier protein to travel through the blood.

• Option IV- Incorrect. Scientifically incorrect, in fact, as increasing bone density (which we’re told Compound X does) would result in a decrease in blood calcium levels.

Answer: B

Content to know: Calcitonin (released by the Thyroid) causes the decrease of blood calcium levels by increasing osteoblast activity (depositing calcium in hydroxyapatite crystals in bone), increasing renal secretion (into the urine) of calcium, and decreasing intestinal calcium absorption. PTH does the exact opposite, being released by the parathyroid.

Explanation: Since we’re simply causing the overexpression of calcitonin, regardless of what group we’re doing it to, then we know that there will be a steeper decrease in blood calcium levels.

Note that A, C, and D are all characteristics of PTH, not calcitonin.

Answer: C

Content to know: Calcitonin decreases calcium concentrations of the blood by either increasing osteoblast activity, increasing renal calcium secretion into the urine, and decreasing intestine absorption of calcium. PTH increases serum calcium concentrations in the exact opposite ways as calcitonin, with help from calcitriol (vitamin D) in the intestine. Don’t confuse calcitriol with calcitonin!

Explanation: Based on the information in the passage, as well as our background content knowledge, we should be able to eliminate B and D pretty quickly because there’s no evidence as such that calcitriol and PTH have any effect on those two.

• B- bone marrow is only responsible for RBC production and helps with the immune system, having no role in calcium levels of the blood.

• D- Hepatic pertains to the liver, and the liver also doesn’t really have any role in maintaining blood calcium levels.

While it may make sense that acting on the parathyroid glands, maybe in some way promoting the secretion of PTH (which we don’t know if this is the case or not!), would decrease serum calcium, we don’t have any evidence of that from the passage.

• A- incorrect, but only because it doesn’t apply to the passage. This isn’t the answer purely because we have a better one in C.

• C- our answer, because we know it’s scientifically correct, answers the question, and is directly mentioned in the passage.

Answer: C

Content to know: None, we basically just need to be able to identify what experiment would prove what the question is proposing.

Explanation: We basically are looking to prove two things, that PTH increases calcium levels of the blood, and does so by interacting with the intestine. First, lets go through the answer choices to see if any of them have any scientifically incorrect information:

• D- scientifically incorrect. calcitonin is not expressed in posterior pituitary tissue. Scientifically incorrect info will never be the answer!

Now let’s make sure that each answer choice answers the question:

• A- incorrect. Increasing the number of osteoclasts would in fact increase serum calcium, but we’re looking for how PTH interacts with the intestines, not bones. There’s probably an answer choice that addresses this.

• B- we’re trying to prove that PTH helps the intestine raise blood calcium levels, where-as this answer choice is basically stating that PTH can raise blood calcium without the help of the intestine. While this is scientifically true, PTH has multiple ways of increasing serum calcium levels, we’re trying to prove that the intestines, when working, can improve calcium levels of the blood, which B does nothing to prove (and if anything would suggest that the intestines are not important in blood calcium levels).

• C- correct. From the passage we know that vitamin D is utilized by the intestines to help absorb calcium into the blood in the presence of PTH, and if inhibiting vitamin D (and therefore PTH activity) doesn’t lead to increases in calcium levels of the blood, that means that PTH and vitamin D are needed to help the intestines raise serum calcium.

Answer: D

Content to know: PTH, released by the parathyroid, increases blood calcium levels, calcitonin, released by the thyroid, decreases them. Hypo = decrease in activity, hyper = increase.

Explanation: A and C should be pretty easy to eliminate, since hypoparathyroidism would lead to decreases in PTH secretion and therefore a decrease in serum (blood) calcium.

• A- decreased PTH would lead to decreased serum calcium.

• C- decreased PTH would lead to decreased intestinal calcium absorption.

This leaves us with B and D as they’re the only two that are scientifically correct with what we’d expect from hypoparathyroidism. To distinguish between the two, we need to pick the best answer choice!

• B- while it’s true that we would expect decreased serum calcium, in order to say that calcitonin secretion would increase, we would have to assume that the thyroid is operating properly. This would be fine if we didn’t have a better answer choice, which D is.

• D- correct, not only because it correctly states what would happen to serum calcium levels, reduced bone resorption is something we can directly predict from decreased PTH release, and requires no consideration of the thyroid gland or calcitonin like B does. D is the better choice!

Your job is to pick the best answer choice, not just any correct answer choice!

Answer: A

Content to know: PTH increases serum calcium levels, Calcitonin decreases serum calcium levels. Osteoblasts, stimulated by calcitonin, cause the storage of calcium into bone from the blood. Osteoclasts, stimulated by PTH, remove calcium from the bone to raise blood calcium levels.

Explanation: If cancer is causing over-active osteoblasts, that means that cancer is causing a decrease in blood calcium levels. This is a question of homeostasis, where something has happened to the body and we’re trying to figure out how the body would react. Decreasing serum calcium concentrations could cause the body to aim to increase serum calcium concentrations, which is done by PTH. PTH would be released to try to get calcium levels of the blood to normal, and calcitonin release would be inhibited (decreased expression).

A is our answer!