Pediatric Examination and Board Review (215 page)

7.
(E)
Routine urinalysis does not include measurement of urine osmolality. Indications for measuring osmolality are a suspected urinary concentration defect as in diabetes insipidus or a need to calculate the urine osmolar gap as described in the answer to question 6.

8.
(C)
The normal range of urine osmolality in a child is 50-1400 mOsm/kg; in an infant it is 50-600 mOsm/kg H
₂
O.

9.
(B)
The aquaporin 2 (water channel) is normally present in the principal cell of the cortical collecting duct and leads to water reabsorption under the effect of vasopressin. In secondary NDI, the expression of aquaporin 2 is decreased and leads to impaired water absorption.

10.
(E)
Secondary NDI can be caused by lithium, colchicine, radiocontrast agents, vinblastine, analgesic nephropathy, and tetracyclines.

11.
(D)
Secondary NDI can occur in chronic renal disease, such as chronic renal failure, chronic pyelonephritis, polycystic kidney disease, medullary cystic disease, uric acid, or calcium nephropathy.

12.
(D)
Acquired or secondary NDI occurs with electrolyte disorders such as hypokalemia and hypercalcemia and is a common metabolic abnormality associated with a urinary concentration defect.

13.
(B)
Acquired NDI can be caused by diseases such as sickle cell anemia, adrenal insufficiency, sarcoidosis, amyloidosis, and nephrocalcinosis. It can also occur with protein starvation.

14.
(B)
NDI can be familial or acquired. There are 2 types of familial NDI. The most common is a mutation in the gene encoding vasopressin receptor V2 in the renal tubular cells, an X-linked recessive disorder. Less common is a mutation in the gene encoding the vasopressin-sensitive water channel aquaporin 2 (AQP 2) in the cells of the renal cortical collecting duct and can be an autosomal dominant or recessive disorder. In acquired NDI, there is usually a decrease in AQP 2 abundance. Vasopressin deficiency would be a central and not a nephrogenic cause of diabetes insipidus.

15.
(C)
Symptoms of NDI include poor feeding, irritability, thirst or eagerness to suck, dehydration, failure to thrive, rarely seizures because of electrolyte imbalance, and constipation. Hydronephrosis and dilated ureters, because of polyuria and consequently increased urine flow, may be present.

16.
(D)
NDI in children can cause poor growth with short stature. Mental retardation has been reported because of recurrent episodes of dehydration with intracranial calcifications. Polyuria leading to frequent trips to the bathroom with consequent interference with learning has been associated with short attention span, distractibility, and hyperactivity. Increased urine flow in NDI can lead to a large bladder (megacystis), not a small bladder (microcystis).

17.
(B)
All of these can lead to increased fluid and free water loss except for low salt intake, which is a therapeutic option for NDI. A high salt intake would lead to increased obligatory free water loss and therefore worsening of the symptoms.

S
UGGESTED
R
EADING

Knoers NVAM, Monnens LAH. Nephrogenic diabetes insipidus. In: Anver ED, Harmon WE, Niaudet P, eds.
Pediatric Nephrology.
6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009.

CASE 123: AN 8-MONTH-OLD BOY WITH VOMITING AND WEIGHT LOSS

An 8-month-old male infant presents to the emergency department with vomiting and weight loss of several days’ duration. He has also had some loose stools. There is no history of fever. A systematic inquiry is noncontributory. He was born at term by normal vaginal delivery with a birthweight of 3.6 kg (50th percentile) and a length of 51 cm (>50th percentile). His development has been normal and his immunizations are reported to be up to date.

On examination you find a thin infant with mild dehydration. He is irritable. His weight now is 7.8 kg (on 5th percentile) and his height is 68 cm (just below the 25th percentile). The remainder of the examination is unremarkable.

Laboratory results are as follows:

BE, base excess; HCO
₃
, bicarbonate; Pco
₂
, partial pressure of carbon dioxide; Po
₂
, partial pressure of oxygen.

SELECT THE ONE BEST ANSWER

1.
This infant has a

(A) high anion gap metabolic acidosis

(B) non–anion gap metabolic acidosis

(C) hypokalemic, hypochloremic metabolic alkalosis

(D) mixed metabolic and respiratory acidosis

(E) none of the above

2.
The urine anion gap is calculated as follows

(A) urine sodium (mEq/L) + urine potassium (mEq/L) − urine chloride (mEq/L)

(B) urine sodium (mEq/L) − urine chloride (mEq/L)

(C) urine sodium (mEq/L) − urine chloride and bicarbonate (mEq/L)

(D) urine sodium (mEq/L) + urine potassium (mEq/L) − urine chloride and bicarbonate (mEq/L)

(E) there is no such thing as a urine anion gap

3.
The urine anion gap in this case is

(A) positive and abnormal

(B) negative and normal

(C) normal

(D) not possible to calculate because of insufficient data

(E) none of the above

4.
This infant has

(A) non–anion gap metabolic acidosis because of gastroenteritis and dehydration (the urine anion gap is suggestive of increased urinary ammonium excretion)

(B) non–anion gap metabolic acidosis because of proximal renal tubular acidosis (the urine anion gap is suggestive of increased urinary ammonium excretion)

(C) proximal renal tubular acidosis because the urine anion gap is suggestive of decreased urinary ammonium excretion

(D) non–anion gap metabolic acidosis because of gastroenteritis and dehydration because the urine anion gap is suggestive of decreased urinary ammonium excretion

(E) none of the above

5.
Another investigation that may be helpful is

(A) spot urine protein-to-creatinine ratio

(B) radiograph of the long bones

(C) renal ultrasound

(D) computed tomography (CT) scan of the head

(E) magnetic resonance imaging (MRI) scan of the brain

6.
This child’s condition can present as

(A) an autosomal dominant disorder inheritance pattern

(B) an autosomal recessive disorder inheritance pattern

(C) primary or secondary to an underlying problem

(D) all of the above

(E) none of the above

7.
The long-term treatment of this child should consist of

(A) dietary sodium restriction

(B) oral sodium citrate or bicarbonate with or without potassium citrate

(C) prostaglandin synthesis inhibitors

(D) all of the above

(E) A and B

8.
All of the following are recognized types of renal tubular acidosis (RTA) except

(A) type I RTA

(B) type II RTA

(C) type III RTA

(D) type IV RTA

(E) none of the above (all of the above are recognized types of RTA)

9.
Which type of renal tubular acidosis causes hyperkalemia?

(A) type I RTA

(B) type II RTA

(C) type III RTA

(D) type IV RTA

(E) none of the above

10.
Which type(s) of renal tubular acidosis cause(s) hypokalemia?