Okay, let's tackle this question. The patient is a 26-year-old male with AML (Acute Myeloid Leukemia) who was started on Doxorubicin-based chemotherapy. Now he's presenting with breathlessness and swelling of the feet. The question is asking which side effect of Doxorubicin caused these symptoms.

First, I need to recall the common side effects of Doxorubicin. Doxorubicin is an anthracycline antibiotic, known for its cardiotoxicity. That's a big one. Other side effects include myelosuppression, alopecia, nausea, and potentially secondary malignancies. But the key here is the symptoms: breathlessness and edema. Those are classic signs of heart failure. So, the cardiotoxicity from Doxorubicin could lead to congestive heart failure, which would present with symptoms like shortness of breath (due to pulmonary congestion) and peripheral edema (from fluid retention due to poor cardiac output).

Now, looking at the options (even though they're not listed here), the correct answer is likely cardiotoxicity or congestive heart failure. The other options might include things like nephrotoxicity, hepatotoxicity, or neurotoxicity. But nephrotoxicity would present with different symptoms like decreased urine output, not edema. Hepatotoxicity might cause jaundice. Neurotoxicity could lead to neuropathy. So, these other options don't fit the presented symptoms.

The clinical pearl here is that anthracyclines like Doxorubicin have a cumulative dose-dependent cardiotoxicity. It's important to monitor the total dose given to patients to prevent this. Also, symptoms of heart failure post-treatment should raise the red flag for cardiotoxicity from Doxorubicin.

So, putting it all together, the correct answer is cardiotoxicity leading to heart failure, which explains the breathlessness and edema. The other options don't align with the symptoms described.

**Core Concept**
Doxorubicin, an anthracycline chemotherapy agent, is notorious for **dose-dependent cardiotoxicity**. It causes oxidative stress and mitochondrial dysfunction in cardiomyocytes, leading to **congestive heart failure (CHF)**, characterized by breathlessness (pulmonary congestion) and peripheral edema (systemic venous congestion).

**Why the Correct Answer is Right**
The patient’s symptoms of breathlessness and lower limb edema point to **CHF**. Doxorubicin-induced cardiotoxicity results from free radical formation and topoisomerase II inhibition in cardiac myocytes, causing myocardial damage. This leads to reduced cardiac output, pulmonary edema, and fluid retention. The cumulative dose (often >550 mg/m²) correlates with risk, but acute or chronic toxicity can occur even within therapeutic ranges.

**Why Each Wrong Option is Incorrect**
**Option A:** *Nephrotoxicity* would present with oliguria, elevated creatinine, or electrolyte imbalances, not edema and breathlessness.
**Option B:** *Hepatotoxicity* typically causes jaundice, elevated LFTs, or coagulopathy, not heart failure symptoms.
**Option C:** *Neurotoxicity* (e.g., peripheral neuropathy) would cause sensory/motor deficits

✓ Correct Answer: B. Hypertrophic cardiomyopathy