WBR0975

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A previously healthy 6 months old boy was brought by his mother to the emergency department for vomiting and pallor. The mother reports that her son was normal at birth and his growth in height and weight meet the normal rate of growth of babies of his age. The baby was breastfed since his birth until a few days ago when the pediatrician advised her to start introducing some soft food in his diet. The mother reports feeding the baby with a small amount of mashed fruits such as bananas or some apple or carrot juice. Which of the following should be avoided in the diet in order to prevent the occurrence of the symptoms in this boy?]]
Answer A	AnswerA::Sucrose
Answer A Explanation	[[AnswerAExp::Sucrose is a disaccharide formed of glucose and fructose. Fructose should be avoided in the diet of patients with fructose intolerance; therefore, this patient should avoid sucrose in his diet.]]
Answer B	AnswerB::Amylose
Answer B Explanation	[[AnswerBExp::Amylose is a spiral polymer made up of D-glucose units. This polysaccharide is one of the two components of starch. Amylose can be included in the diet of patients with fructose intolerance.]]
Answer C	AnswerC::Cellulose
Answer C Explanation	[[AnswerCExp::Cellulose is a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants. Humans can digest cellulose to some extent; however, it mainly acts as a hydrophilic bulking agent for feces and is often referred to as "dietary fiber". Cellulose can be included in the diet of patients with fructose intolerance.]]
Answer D	AnswerD::Lactose
Answer D Explanation	[[AnswerDExp::Lactose is a disaccharide formed of glucose and galactose. Lactose can be included in the diet of patients with fructose intolerance.]]
Answer E	AnswerE::Maltose
Answer E Explanation	AnswerEExp::Maltose is a disaccharide formed of two molecules of glucose. Maltose can be included in the diet of patients with fructose intolerance.
Right Answer	RightAnswer::A
Explanation	[[Explanation::The baby presents with nausea, vomiting and pallor following ingestion of fruits for the first time in his life. This is a typical presentation of fructose intolerance, an autosomal recessive genetic disease of fructose metabolism. Fructose intolerance is caused by a deficiency in the enzyme aldolase B, leading to the accumulation of fructose 1-phosphate. The accumulation of fructose 1-phosphate inhibits glycogenolysis and gluconeogenesis due to the depletion of phosphate. The symptoms of fructose intolerance include hypoglycemia, vomiting, jaundice and cirrhosis if fructose is not eliminated from the diet. The treatment of this condition is the exclusion of fructose, sucrose (a disaccharide formed by glucose and fructose) and sorbitol from the diet. Shown below is a diagram depicting fructose metabolism. Educational objective: Fructose intolerance is an autosomal recessive disease caused by the deficiency in the enzyme aldolase B. Patients with fructose intolerance should exclude fructose, sucrose and sorbitol from their diet. Reference: First aid for USMLE step 1, 2013. Page 104. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Fructose metabolism, WBRKeyword::fructose intolerance
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::