Özet: The most likely diagnosis is HIV Infection. Explanation for HIV Infection: The patient presents with a classic constellation of symptoms indicative of advanced HIV infection, specifically progressing to AIDS or an AIDS-defining illness: * Persistent low-grade fever and night sweats: Common systemic symptoms in chronic HIV infection. * Unintentional weight loss (10 pounds in 2 months): Often referred to as "wasting syndrome," a significant sign of chronic illness and immune suppression. * Recurrent boils and slow-healing skin infections: Suggests impaired immune function, making the patient more susceptible to opportunistic bacterial skin infections. * Painful, swollen lymph node (left armpit): While lymphadenopathy can occur in various conditions, persistent, generalized lymphadenopathy (PGL) is common in HIV infection, and an enlarging node can also indicate opportunistic infection or lymphoma in HIV-positive individuals. * Persistent oral thrush (oral candidiasis): This is a hallmark opportunistic infection in individuals with compromised immune systems, particularly HIV. Its resistance to over-the-counter treatments further points to an underlying immune deficiency. * Past medical history of hepatitis B: HIV and hepatitis B coinfection is common, as they share similar transmission routes. * Sexual history with multiple partners and inconsistent condom use: This is a significant risk factor for HIV transmission. Why other options are less likely: * 1. Chronic Fatigue Syndrome: While fatigue is a primary symptom, the presence of fever, night sweats, significant weight loss, recurrent infections, oral thrush, and lymphadenopathy goes beyond what is typically seen in Chronic Fatigue Syndrome. These are signs of active systemic illness and immune compromise. * 3. Lymphoma: Lymphoma is a *possible complication* of advanced HIV infection (AIDS-defining malignancy) and could explain the lymphadenopathy, fever, night sweats, and weight loss. However, the recurrent skin infections and, most notably, the persistent oral thrush (an opportunistic infection) are not directly explained by lymphoma itself. HIV infection provides a more overarching explanation for all the symptoms, with lymphoma being a potential secondary diagnosis. * 4. Tuberculosis: Tuberculosis can also cause fever, night sweats, weight loss, and lymphadenopathy. However, oral thrush and recurrent skin infections are less typical primary manifestations of TB. While TB is a common opportunistic infection in HIV, HIV infection is the more fundamental diagnosis that explains the broad immune suppression. * 5. Reactivation of Hepatitis B: While fatigue can be a symptom of hepatitis B, and he has a history, the extensive list of other symptoms (persistent fever, night sweats, significant weight loss, recurrent infections, oral thrush, lymphadenopathy) is not characteristic of isolated hepatitis B reactivation. These symptoms strongly point to immune deficiency, which hepatitis B alone does not cause. Conclusion: The combination of risk factors (sexual history), systemic symptoms (fever, night sweats, weight loss, fatigue), and multiple opportunistic infections (oral thrush, recurrent boils) strongly points to advanced HIV infection. This diagnosis would then explain the patients profound immune suppression leading to these diverse clinical manifestations.