Final answer:
The presentation of malaise, periorbital edema, and smoky-colored urine in a child post streptococcal infection suggests poststreptococcal glomerulonephritis, a condition resulting from immune complex deposition in the kidneys.
Step-by-step explanation:
A 4-year-old girl presenting with malaise, periorbital edema, and smoky-colored urine after a strep throat infection should make you think of poststreptococcal glomerulonephritis (PSGN). PSGN is a type of acute glomerulonephritis that occurs as a sequela after an infection by particular nephritogenic strains of S. pyogenes, typically 1-3 weeks following an untreated or inadequately treated pharyngeal strep infection. The disease mechanism involves immune complex formation between streptococcal antigens and antibodies, which are then deposited in the kidneys, leading to inflammation and damage.
In this clinical scenario, a lowered serum complement level and an elevated antistreptolysin O (ASO) titer would support the diagnosis of PSGN. Acute rheumatic fever, Henoch-Schönlein purpura, and IgA nephropathy are other possible differential diagnoses but are less likely given the acute onset following a streptococcal infection and the described symptoms in this young patient.