Exam 3: Biochemistry

A 13-year-old boy is brought to the office due to gait instability and pruritic skin rash for the past several weeks.  His mother reports that he has also been irritable and had loose stools during this time.  The patient's childhood development has been unremarkable except for several episodes of similar skin rash that resolved spontaneously.  Examination shows scaly, erythematous skin lesions in sun-exposed areas and cerebellar ataxia.  Laboratory evaluation shows increased levels of neutral amino acids in the urine.  This patient's symptoms would most likely respond to which of the following supplements?

Two graphs illustrating the transport rate of solutes across the plasma membrane are shown on the slide below. Which of the following best explains the difference in the shape of the curves?