Okay, let's tackle this question step by step. The patient is a 9-year-old boy with symptoms pointing towards post-streptococcal glomerulonephritis (PSGN). The key points here are the history of skin lesions two weeks prior, now presenting with decreased urine output, cola-colored urine (which suggests hematuria), edema, and hypertension. The question is asking about the antibiotic of choice to limit the spread of the nephritogenic organisms.

First, I need to recall what causes PSGN. It's typically a complication following a streptococcal infection, either pharyngeal or skin (like impetigo). The nephritogenic strains involved are Group A β-hemolytic streptococci. The treatment here isn't to treat the current glomerulonephritis but to eradicate the remaining streptococcal organisms to prevent further spread among others.

The standard antibiotic for this scenario is penicillin. Since the infection is likely to be in the skin (given the history of skin lesions), oral penicillin V might be used, but intravenous penicillin G is often preferred for more effective eradication. However, the question is about the drug of choice to limit spread. Penicillin is the first-line because it effectively targets Group A streptococci. If the patient is allergic, alternatives like erythromycin or clarithromycin are used, but the best choice is penicillin.

Looking at the options, assuming the options include penicillin (maybe as penicillin G or V), that's the correct answer. The other options would be antibiotics that don't target Group A strep effectively. For example, cephalosporins might be used in some cases but aren't the first line here. Amoxicillin is a penicillin derivative, so if that's an option, it could be a distractor. But the key is penicillin's role in eradicating the streptococcal antigen to prevent PSGN spread.

Wait, but in PSGN, the treatment of the streptococcal infection is to prevent further cases, not to treat the glomerulonephritis itself. So the antibiotic use is prophylactic in this context. The duration is usually 10 days to ensure complete eradication. So the correct antibiotic is penicillin, either G or V, depending on administration route.

Now, checking the wrong options: If the options include cephalosporins, they are not first-line. If there's an option like cefazolin, that's a cephalosporin and not the best choice. If there's a macrolide like erythromycin, it's a second-line. Clindamycin isn't effective for Group A strep. So the key is identifying penicillin as the drug of choice here.

**Core Concept**
Post-streptococcal glomerulonephritis (PSGN) results from immune complex deposition following Group A β-hemolytic streptococcal (GAS) infections. Treatment with antibiotics aims to eradicate remaining streptococcal antigens and prevent further spread, not to treat the glomerulonephritis itself.

**Why the Correct Answer is Right**
Penicillin (e.g., penicillin G or V)

✓ Correct Answer: B. Penicillin