A 17-year-old boy who was recently diagnosed with type 1 diabetes mellitus is found unresponsive at home.  His family reports that the patient did not feel good last night and skipped dinner, but he still took his usual dose of insulin.  Paramedics find that his fingerstick blood glucose is 32 mg/dL and immediately administer intramuscular glucagon.  Glucagon binds to a transmembrane receptor in hepatocytes, which promotes binding of intracellular GTP to a specific receptor-associated protein.  This leads to further downstream signaling and rapid metabolic changes within hepatocytes, including a rapid decrease in intracellular glycogen stores and release of glucose into the blood.  Which of the following is the most likely mediator responsible for these effects?

A 6-year-old girl is brought to the office for evaluation of short stature.  Her parents have noticed that she bears little resemblance to her 2 older siblings.  The patient’s mother and father are 172.5 cm (5’8″) and 183 cm (6′) tall, respectively.  On physical examination, the patient’s height is 3 standard deviations below the mean for her age.  Other findings include low-set ears, a high arched palate, a webbed neck, and cubitus valgus.  Chromosomal analysis reveals a 45,XO karyotype.  The patient is started on a medication to improve growth and normalize her height.  Which of the following intracellular pathways is stimulated by the medication used in this patient?

A 69-year-old woman with Alzheimer disease is brought to the emergency department after getting lost while taking a walk in her neighborhood.  Her son has been unable to contact the patient for the last 2 days, and today the police found her wandering in a park.  The patient says that she drank water from a park fountain but has not had anything to eat for over 24 hours.  On physical examination, she is mildly confused with dry mucous membranes.  Laboratory studies show a blood glucose level of 92 mg/dL.  Which of the following hormones binds to an intracellular receptor to help maintain this patient’s laboratory findings within the normal range?

An investigator is studying weight regulation using experimental mouse models.  Knockout mice are created with a homozygous mutation in the gene coding for the leptin receptor.  This mutation prevents the receptor from binding leptin and initiating its normal signaling cascade.  The knockout mice are allowed to feed at will, and their body mass index (BMI) and serum leptin levels are measured and compared with control mice.  On the graph below, area C represents the normal relationship between BMI and leptin in a control mouse.

Which of the following areas represents the expected relationship between BMI and serum leptin levels in a leptin receptor mutant mouse?

A 54-year-old woman is evaluated for progressive constipation, anorexia, and a 5.4-kg (12-lb) weight loss over the past several months.  Physical examination is unremarkable.  Stool guaiac test is positive, and a colonoscopy is performed.  An exophytic mass is identified in the sigmoid colon.  The patient undergoes a left hemicolectomy, and histopathology of the surgical specimen is positive for adenocarcinoma.  Molecular testing of the cancer cells reveals a mutation in the KRAS gene that results in constitutive activation of the Ras protein.  Under normal circumstances, this protein is only active when bound to which of the following substances?

Molecular biologists studying signal transduction apply an agent to human cells that activates G-protein- dependent phospholipase C.  Which of the following intracellular substances is most likely to increase immediately after exposure to this agent?

A 14-year-old boy is brought to the emergency department due to excessive urination and thirst.  He has lost 4.5 kg (10 lb) in the last 3 weeks.  The patient has no past medical problems.  His father has type 1 diabetes mellitus.  Physical examination shows dry mucous membranes.  Laboratory studies reveal blood glucose of 455 mg/dL, normal anion gap, and hemoglobin A1c of 11.3%.  The patient is diagnosed with type 1 diabetes, and treatment with insulin is initiated.  In addition to lowering blood glucose, insulin increases glycogen synthesis in hepatocytes.  Activation of which of the following molecules most likely promotes this metabolic effect?

Researchers lyse human cells and isolate a specific messenger RNA template using gel electrophoresis.  Reverse transcription polymerase chain reaction is then used to synthesize complementary DNA (cDNA) from the RNA template.  Next, the cDNA is modified into an expression vector containing an optimized bacterial promoter, ribosomal binding site, and terminator sequence.  After insertion of the vector into appropriate bacterial hosts, the transformed bacteria are cultured in a bioreactor and produce large quantities of a protein containing a domain that binds to a specific DNA sequence.  This protein is most likely the receptor for which of the following hormones?

A research scientist develops an agent that specifically blocks the interaction of inositol triphosphate with its intracellular receptor.  A study is then performed in which vascular smooth muscle cells are divided into 2 groups: an experimental group treated with the receptor blocker and an untreated control group.  Both groups are exposed to phenylephrine.  Compared to the control cells, decreased activity of which of the following enzymes is most likely to be observed in the experimental cells?

Researchers are investigating the relationship between glucose transport proteins and insulin concentration in different types of tissues.  Experiments in which cells are taken from various tissue samples and exposed to increasing concentrations of insulin are performed.  The number of surface glucose transporters are then measured.  The results from 2 cell samples are plotted on the graphs below.

Which of the following cell types are most likely represented in cell samples A and B, respectively?

A researcher is studying the structure of different hormone receptors.  Receptor proteins are isolated and purified from a homogenized tissue sample.  Detailed structural analysis shows that one of the proteins contains a 30-amino acid motif coordinating a zinc molecule, as shown in the image below.

A receptor for which of the following hormones was most likely isolated in this experiment?

A 23-year-old man is brought to the emergency department by paramedics following a motor vehicle accident.  He was an unrestrained passenger in the front seat.  Several days after hospitalization, his fluid volume and plasma osmolarity are measured and illustrated in the image below (solid line, normal; dotted line, patient).

Which of the following conditions is most likely to cause the findings shown in the image?