A highly interesting medical problem

A brief summation of the events that led to this post: despite my schedule being largely saturated, I stumbled upon some time to, for lack of a better word, kill.  By dint of this, I found myself on Facebook; somehow, I chanced upon the Faculty of Medicine page.  I scrolled down, found the below posting, and proceeded to attempt to work it out.

Question of the Week #1

An 11 month old male is evaluated again by his pediatrician for poor growth. Due to intolerance of cow’s milk based formula as a young infant, he has been feeding exclusively with soy formula and a limited variety of table foods. Currently, the infant’s length is less than the 5th percentile, while his weight at the 10th percentile for his age. On examination, you note a widely open fontanelle with a thin occipital bone that depresses slightly and recoils rapidly. There is prominence of the costochondral junction at the anterolateral chest, and the femurs are somewhat bowed bilaterally. Laboratory evaluation demonstrates normal electrolytes and BUN and creatinine, but a serum phosphorus of 1.9 mmol/L with serum calcium of 10.6 mg/dL. The urinary excretion of phosphorus is measured, and the fractional excretion of phosphorus is calculated to be 25% (normal 5-20%).

Which of the following is the most likely etiology for this patient’s hypophosphatemia?

A. Hypoparathyroidism
B. Primary vitamin D deficiency
C. Deficient intake of phosphorus from low phosphorus soy formula
D. Increased cellular uptake of phosphorus from refeeding syndrome
E. An X-linked defect in renal tubular reabsorption of phosphate

Let us commence with the first option – hypoparathyroidism.  As its name suggests, this is a decrease in the function of the parathyroid glands, and thus, a decrease in the amount of PTH (responsible for increasing the concentration of calcium found in one’s blood) a person harbors.  The aforementioned decrease leads to many symptoms, none of which are explicitly mentioned in the observations given.  As could be expected, one tests for hypoparathyroidism by measuring calcium and PTH as they appear in the blood.  cAMP may also be measured, but this is typically done after one’s having been given an intravenous dose of PTH.  In either case, PTH would have been mentioned.

A. Hypoparathyroidism
B. Primary vitamin D deficiency
C. Deficient intake of phosphorus from low phosphorus soy formula
D. Increased cellular uptake of phosphorus from refeeding syndrome
E. An X-linked defect in renal tubular reabsorption of phosphate

On to B).  I will not even bother to elaborate – this seems to be the answer.  Vitamin D deficiency often results in softened bones, which would explain the peculiar behavior of the above-mentioned occipital.  Additionally, rickets can be caused by a child lacking D – it is characterized by impeded growth and deformity of the bones, as is present in this infant.  Additionally, rickets has been associated with low dietary calcium intakes; the infant has been fed exclusively with soy formula.

So, is that the answer?  Let’s continue.

We can effectively rule out C), as the final sentence of the question eradicates that possibility.

A. Hypoparathyroidism
B. Primary vitamin D deficiency
C. Deficient intake of phosphorus from low phosphorus soy formula
D. Increased cellular uptake of phosphorus from refeeding syndrome
E. An X-linked defect in renal tubular reabsorption of phosphate

There was absolutely no mention of the infant’s ever having been fed in an atypical manner, ergo there isn’t a cause for refeeding syndrome.  Eliminate D).

A. Hypoparathyroidism
B. Primary vitamin D deficiency
C. Deficient intake of phosphorus from low phosphorus soy formula
D. Increased cellular uptake of phosphorus from refeeding syndrome
E. An X-linked defect in renal tubular reabsorption of phosphate

Now, E).  It could be phosphate diabetes, but primary vitamin D deficiency seems to be the answer.  You see, the serum calcium connotes that he has mild hypercalcemia; the urinary excretion matches E.  As such, it’s a tad grievous to establish a rigid conclusion.

To bee or not to bee?  That is the question.

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