A 9-year-old boy is brought with history of decreased urine output, cola colored urine and swelling of the face and hands of 2 days duration. He is hypertensive, has a puffy face and pitting edema of the lower limbs. He has history of skin lesions 4 weeks earlier. A diagnosis of post streptococcal glomerulonephritis is made. If an antibiotic is to be used to limit the spread of the nephritogenic organisms, the drug of choice is?