ANSWERS OF 'TO MATCH'

ANSWERS OF 'TO MATCH'

1. I. X-linked hypophosphataemic rickets

In this condition, a mutation in the PEX gene on the X chromosome

causes an isolated defect in phosphate reabsorption. This low rate of tubular reabsorption of phosphate results in hypophosphataemia, and therefore rickets develops. Typical radiological changes in rickets include cupping and fraying of the metaphyseal regions. Other features include delayed dentition. The normal 2 5-hydroxy vitamin D and P T H levels, together with a low phosphate level, provide the clue to the diagnosis.

2. F. Vitamin D deficiency rickets

This child has vitamin D deficiency leading to rickets. The low 25-hydroxy vitamin D level provides the clue to the diagnosis. This child requires a careful detailed history of diet and sun exposure. Rickets typically causes genu varum (bowed legs) in children under 2 years of age, and genu valgum (knock-knees) in children over 2 years of age.

3. G. Primary hypoparathyroidism

Low parathyroid hormone levels result in hyperphosphataemia and

hypocalcaemia. The seizures are likely to be secondary to hypocalcaemia. In this case the hypoparathyroidism is likely to be due to Di George syndrome (congenital abnormality of the third and fourth brachial arches leading to hypoparathyroidism, absent thymus, dysmorphic features and congenital cardiac disease).

4. L. Proximal renal tubular acidosis

Proximal (type 2) renal tubular acidosis is caused when there is reduced proximal tubular reabsorption of bicarbonate. Norm ally 85% of the filtered bicarbonate is reabsorbed in the proximal tubule. The distal tubule cannot fully compensate for this load, so considerable amounts of bicarbonate are lost in the urine, and the serum bicarbonate level falls, resulting in a metabolic acidosis. The urine can still be acidified because distal hydrogen ion secretion is normal. Thus the acidic urinary pH is the clue that this is proximal renal tubular acidosis. Patients may present with faltering growth or vomiting.

5. C. Distal renal tubular acidosis

Distal (type 1) renal tubular acidosis is caused when the distal nephron cannot secrete hydrogen ions, thus preventing the reabsorption of the final 15% of bicarbonate. This causes a metabolic acidosis. However, the urinary pH cannot be reduced below about 5.5-6.0, even in the presence of severe systemic acidosis. If there is systemic metabolic acidosis, the body would usually try to compensate by producing acidic urine. In this case the urine cannot be acidified, which provides the clue to the diagnosis

of distal renal tubular acidosis. Patients may present with urinary stones or nephrocalcinosis.